GEORGE BALL WAS the undersecretary of state when the United States was sucked into Vietnam like a long piece of thread drawn into a vacuum cleaner. Ball was the highest American official in those days who fought against military involvement in Vietnam. Now an investment banker, Ball is watching the United States fight against the suction in El Salvador.

He likens the spectacle to a song, or the soundtrack of a movie: "The music and words seem to be almost a plagiarization. I have the feeling we've heard it all before, but in another setting."

No, no, reply officials of the Reagan administration -- El Salvador is not Vietnam. Secretary of State Alexander M. Haig insists that any attempt to draw a parallel between the two "is a terrible distortion of reality and one which overlooks a number of fundamental differences."

Curiously, Ball and Haig are both right. El Salvador is profoundly different from Vietnam, but it is also eerily familiar. The setting has changed, but it looks like the same old movie.

The Reagan administration involuntary evokes Vietnam in almost every public statement it makes about El Salvador. This is an East-West test. The rebels are controlled from Nicaragua or Cuba or Moscow. If we don't stop them now the other dominoes will begin to tumble. Oh, yes, and please trust us. We have the real dope. We know.

But for Americans El Salvador is different from Vietnam in one overriding respect: It comes to us after Vietnam. We were willing to go to war in Vietnam because the president, his State Department and his Pentagon told us we had to fight to protect the national interest. Not this time.

According to a recent Gallup Poll, the American public opposes sending U.S. troops to El Salvador by 89 percent to 8 percent. Even sending military supplies or advisers is opposed by substantial majorities.

In Vietnam we were more malleable. At the end of 1965, for instance, after the first U.S. combat forces had gone to fight in Vietnam, Gallup found that the public still supported the war effort by 3 to 1.

Secretary Haig, of course, notes other distinctions between El Salvador and Vietnam. Pressed on the question last week, this was his answer:

"First and foremost is the difference with respect to the strategic importance of Central America . . . to the United States today . . . This is a vitally important region . . . The outcome of the situation there is in the vital interest of the American people and must be so dealt with."

In other words, in Haig's opinion the country where America lost 57,639 lives and squandered $130 billion was not of vital importance to the United States; but El Salvador is.

And what is the "reality" Haig thinks is being distorted? In his view it is a case of external aggression against El Salvador. In Haig's words (speaking of the leftist Salvadoran rebels): "They're being commanded, controlled, and run externally -- completely. They are being armed and trained externally, and they are not espousing the wishes of the people of Salvador." There goes the old movie again.

What about the history of El Salvador? What about the story of the 14 families that ran the place like a family farm for decades? What about the supposedly anti-communist Christian Democratic politicians in El Salvador who have joined the rebels? What about the accusations that the army and police are responsible for most of the deaths and arbitrary violence? The list of "what abouts" could be long one indeed, but the movie skips over those questions, leaves them unasked -- as the old one did for so long.

Perhaps modern life is a series of movies, some new, some reruns. While the Vietnam movie certainly seems to be running again at times, whoever is up there in the projection booth is playing with the machinery. This time the movie is running much faster than it did in the l960s.

At the beginning in Vietnam, only a few people paid much attention. The networks didn't start their half-hour evening news programs until 1962, well after the first U.S. military advisers took up the cudgels against the Vietcong, and they didn't devote much air time to this far-off counterinsurgency effort. Of course no one knew then what was coming. The thread was already caught in the pull of the Vietnam vacuum, but almost no one realized it. The first time around this was a new movie, and the plot had to unfold one scene at a time.

Now everyone thinks they know how the movie ends, and no one wants it to get that far.

So the networks have camera crews scurrying around Salvador like ants on a picnic blanket. Every night the network correspondents share a one-hour satellite "feed" to the states -- instant communications, not shipping film to Hong Kong or Bangkok the way they did in Vietnam, even at the height of the war. Congressional committees are already giving administration officials skeptical, even hostile, grillings of a kind they didn't get until late in the Vietnam war.

Government officials want to rewrite the ending to this movie, too. Privately, it is reliably said, Alexander Haig rules out the use of American ground forces in Central America as definitively as any of the members of Congress who are shouting out against the possibility in public. Professional soldiers at the Pentagon show no appetite for a military adventure in the Central American mountains. Civilian officials at the Defense Department are impressed by the professional military's unwillingness to contemplate any new use of American armed force that is not enthusiastically supported by the public.

Stop the movie; we want to get off.

There are many other differences and similarities between El Salvador and Vietnam. Consider some differences:

Vietnam is nearly the size of California, with 47 million inhabitants (including both north and south). El Salvador, the size of Massachussetts, has just 3 million citizens.

Vietnam is on the other side of the world, and it shares a South Asian peninsula with Cambodia, Laos and Thailand -- hardly a strategic location. El Salvador is just 1,400 miles from Florida and 800 miles from the Panama Canal, still a vital link for American commerce and military logistics. It is surrounded by similarly impoverished countries long ruled by feudal oligarchies and ripe for insurgencies of their own. Guatemala, the biggest and richest nation in Central America, already has one; and Nicaragua already has a radical leftist regime because its insurgency prevailed. Cuba, nearby, is eager to help leftist rebels in all of these countries, and appears to have weapons and materiel that they can use.

Vietnam before 1975 was an artificially divided country whose northern half was ruled by a ruthlessly efficient -- and nationalistic -- totalitarian regime. Reunification of the country was the overriding preoccupation of that regime, which could get all the arms and equipment it needed from China and the Soviet Union. Vietnam's southern half was ruled by a Catholic family whose connections to the United States were stronger than its nationalist credentials. Thanks to the superpower patrons of both sides and the large populations in both halves of the country, the Vietnam war involved hundreds of thousands of troops on both sides -- not to mention the Americans involved.

In El Salvador there are only about 32,000 armed troops in the continuing civil war. The leftist insurgents have friendly almost-neighbors in Nicaragua (they are separated by about 30 miles of Pacific Ocean or 70 miles of Honduran territory) and more powerful friends in Cuba. But there is no prospect that a Salvadoran civil war could ever assume the proportions of the Vietnam war. The soundstage for this movie is just too small.

There is no American Armed Forces Network radio station in El Salvador, probably because there are only about 50 U.S. military men in the country. When John F. Kennedy entered the White House in 1961, there were already 3200 American advisors in South Vietnam.

Then ponder other similarities:

The American government has committed itself rhetorically to an outcome in El Salvador, just as it committed itself to an outcome in Vietnam, long before it was clear that this outcome could be achieved. "We will do whatever is prudent and necessary to ensure the peace and security of the Caribbean area," President Reagan said last month. Haig has been more specific, committing the United States to do "whatever is necessary" to head off a rebel victory in El Salvador.

It was the same in Vietnam. As early as the Truman administration, American policymakers were insisting on the strategic importance of Indochina to the United States. The Tonkin Gulf resolution formally declared America's commitment to stop "aggression."

Reading through old speeches one is reminded of a form of rhetoric that has been resurrected for El Salvador.

"I believe, and I am supported by some authority, that if the communists are not checked now the world can expect to pay a greater price to check them later." So said Lyndon B. Johnson in January 1967, speaking of Vietnam.

"There is no question that the decisive battle for Central America is under way in El Salvador . . . If after Nicaragua El Salvador is captured by a violent minority, who in Central America would not live in fear? How long would it be before major stratetgic United States interests -- the Panama Canal, sea lanes, oil supplies -- were at risk?" So said Thomas O. Enders, assistant secretary of state for Inter-American affairs, just last month.

Because Americans like to act, rhetoric has not stood alone, just as it didn't in Vietnam. We already have military advisers on the scene in El Salvador -- though we have renamed them "trainers." Trainers, advisers, whatever -- they are our tribute, our tangible contribution meant to make our commitment look good.

Of course advisers in Vietnam proved inadequate to the task at hand, a task that wasn't carefully measured before the commitment was made. In El Salvador, again, the advisers don't seem sufficient to stem the rebel tide. Haig now bridles at suggestions that with all the help we've already given, our side in El Salvador should have done better in the civil war.

"Nobody had hoped they would go any better," Haig said of the government's forces in a recent interview with The Post. "That's the one thing I want to disabuse you of. All we did in the first stages of our assistance program where we sent 50 guys and a few program where we sent 50 guys and a few helicopters and $30 million more to El Salvador was to help to slow down a tide that we inherited. And nobody deluded himself that this was going to solve El Salvador . . ."

Our side also looks weak again. In Vietnam our side looked so weak in 1963 that the United States encouraged a coup against Ngo Dinh Diem, the Vietnamese patriot on whose behalf we made our original commitment to South Vietnam. That left us with no leader on our side, just army officers, against Vietnamese nationalists led by Ho Chi Minh. For 12 years we tried to transform those generals into popular political leaders, with no real success.

In El Salvador our side is an army junta nominally led by a civilian politician, Jose Napolean Duarte. Duarte was once undeniably popular, but his standing now seems problematical. Reports from El Salvador suggest that he may be ousted after the March 28 elections (which the rebels are boycotting and trying to subvert) and replaced by a rabid right-winger who -- is this in the script? -- got his start as an army officer.

A final similarity: The actual situation "on the ground" is baffling. El Salvador's history -- like Vietnam's -- did not begin the moment the United States took an intense interest in the place. Salvadoran actors are playing out a Salvadoran drama that isn't part of our movie. There are ancient emnities, a well-established social and economic order, ol d traditions of leadership and much else in El Salvador that can't be explained away by "external aggression." Who and what is our side fighting for?

Our movie skipped over these tough, fundamental questions in Vietnam. The rerun is skipping over them, too.

Walt W. Rostow was Lyndon Johnson's national security adviser during the Vietnam war. He is now a professor at the University of Texas. Rostow also sees similarities between Vietnam and El Salvador.

"I'm not an expert on El Salvador," Rostow said in a recent conversation in Austin. Nevertheless, he went on, this moment has a familiar quality. "It's the same bunch" of "left-intellectuals" and journalists who undermined the Vietnam war effort who are at work now undermining our efforts to help Salvador, he said. "I fear for my country," he added, complaining of the "knee-jerk" reaction he perceived in the media that "romanticizes terrorists" and denegrates American policy.

George Ball sees a similarity of a different kind. "We are making essentially the same mistake in thinking that all local situations must be resolved in terms of the East-West conflict," Ball said. "What's going on in Salvador is only tangentially related to the Soviets," he added. "This is not a problem that can be solved by military means. It's total nonsense to say that it is."

Another perspective -- the perspective from television that helped create the old film -- is offered by Gary Shepard of CBS, who covered Vietnam and just returned from three weeks in El Salvador. "The most vivid recollection I've got that ties these two stories together is the Huey helicopter," Shepard said. In Vietnam that warhorse chopper landed beside rice paddies; in Salvador it's sugar cane fields -- same helicopter.

Does the local life and culture seem as opaque and foreign as it did in Vietnam? "I think we have a better handle on it in Salvador," he replied. He said it wasn't too hard to see how years of oligarchic rule by a few families had helped radicalize the peasantry and lay the groundwork for civil war.

"It really is a civil uprising to a great degree . . . a pure version of a civil war," Shepard said. Cuba and the Soviet Union can't be helping very much, he added.

According to Shepard, the insurgents are better at public relations than the government. Whereas it is easy to visit rebel forces, interview their commanders and watch them in action, the Salvadoran army doesn't accommodate the TV crews, he said.

Is CBS in New York receptive to material from El Salvador? Oh, yes, Shepard replied, they took everything he could send during a recent three-week assignment. The network still likes "bang-bang" (that is, battlefield) coverage, as it did in Vietnam, he said, "but they want bang-bang with a message" -- explanations of what's really going on on the ground.

Gen. Samuel V. Wilson was in the thick of the Vietnam war in its early stages. Later he was the U.S. military attache in Moscow. Later still he was director of the Defense Intelligence Agency. He's now retired in Rice, Va.

"We sort of came wretching and vomiting out of our Vietnam experience saying 'Never Again!' " Wilson said the other day. "We've sort of spooked ourselves."

Like Secretary Haig, Wilson believes that America's vital interests are more clearly at risk in Central America than they ever were in Vietnam. He thinks that despite Vietnam, Americans could be persuaded to support "reasonable steps in this area" to secure those interests, provided they understood what was involved.

"But I don't see evidence that we learned a helluva lot from our experience in Southeast Asia," Wilson said. The main lesson he learned, Wilson went on, is that a man in one of these third-world countries racked by internal strife "has got to be able to see fairly clearly what his options are . . . If he sacrifices himself, what are his possible gains?"

The man in the rice paddy, like the man in the sugar cane field, wants security, social justice, educational opport unity for his children, rudimentary health care and some basic economic opportunity, Wilson suggested. "You have to look at your programs and see how well you satisfy these five," he said. "You start dealing with these first."

Americans are too quick to make others' problems their own, Wilson remarked. "Hell, this is a much bigger problem for Mexico than it is for us." He suggested the United States encourage Mexico to take initiatives in El Salvador that we could endorse and support. "We'd be so much better off getting somebody else into this instead of playing the dominant Yankee role from the North."

Secretary Haig has already looked ahead to the end of the movie. He does not see a light at the end of the tunnel; he sees refugees.

"Just think what the level (of illegal Central American immigration into the United States) might be," Haig said recently, "if the radicalization of this hemisphere continues with the only alternative a totalitarian model in one state after the other. Why, it will make the Cuban influx look like child's play."

Will there also be boat people? Please, somebody change the script.