To ask yourself whether El Salvador is going to be "another Vietnam" is to invite cold comfort. History does not repeat itself, precisely; to begin with, El Salvador has no open border with a thoroughly communist "North El Salvador," openly supported by the Soviet Union.

For a whole lot of other unreassuring reasons -- demographic, geographic, topographic -- the answer then is: No, El Salvador is not Vietnam.

But when you have broken down large parts of the Vietnam analogy some enormously unsettling similarities remain in the Haig/Reagan approach to El Salvador. Even the administration's protestations of what it will not do have a familiar ring: There will be no American combat troops; American boys, to paraphrase Lyndon Johnson in 1964, will not fight wars that El Salvadoran security forces are supposed to fight.

Instead, we are going to "strike at the source" -- the external support through Nicaragua, from the Communist bloc, Vietnam, Ethiopia, the radical Arabs, with Cuba as the focal point. A blockade of Cuba is not ruled out. "Striking at the source," you will remember, is what we were doing in Cambodia, over Hanoi, in the mining of Haiphong.

Even the trappings -- the look and sound of things, the language -- are the same: a diplomatic flying circus, to shape up the allies in Europe and friendly Central Americans for a square-off against the Soviet Union's global designs with El Salvador as the test; a "White Paper" to whip up American public support; mountainous briefing papers for Congress to document the communist origins of the arms flow to El Salvadoran leftists.

"Agression," plain and simple, is the charge. And so, "a line must be drawn," Remember the dominoes!

But why El Salvador? And why just now? I'm not suggesting there's no crisis in El Salvador, or that the Soviets and the Cubans and the rest do not have a considerable hand in it. But the crisis is, if anything, less intense now than it was in the days just before the new administration came to power.Then a full-scale leftist guerrilla offensive was under way; United States intelligence had already well established a growing flow of more sophisticated arms through Nicaragua.

The Carter administration responded with emergency shipments of military aid, indluding a half-dozen troop-carrying helicopters and "lethal" weaponry. But -- and this bears careful noting -- the El Salvadoran government security forces had managed to beat backthat offensive on its own, before the new war material had arrived, according to officials dealing with the problem at the time.

This doesn't mean, of course, that the junta can retain the upper hand, while the leftists regroup for another try and continue to receive heavy shipments of arms from outside. The Pentagon apparently thinks not. This leads logically to an effort to stop this flow of outside support by loud threats of "striking at the source."

Conceivably, this will do the trick. Cuba's Castro has a reputation for prudence, for moving "gingerly," as one expert puts it. But the odds on choking off all outside support and mopping up the leftist forces strike most experts as exceedingly remote.

And that, a lot of experts agree, is because there is a lot more to the troubles of El Salvador than the external threat. In his presentation to allied governments, Secretary of State Haig made much of the point that the El Salvadoran junta is a "coalition, headed by a true Christian democrat, Napoleon Duarte." He made very little of Duarte's own definition of what's going on in El Salvador:

"This is a history of people starving to death, living in misery. For 50 years the same people had all the power, all the money, all the opportunites.

"Those who did not have anything tried to take it away from those who had everything. But there were no democratic systems available to them, so they have radicalized themselves, while resorting to violence. And of course this second group, the rich, do not want to give up anything, so they are fighting."

Now, if that is what the El Salvador blood bath is all about, it does not seem to me to lend itself as a prime candidate for an East-West test of wills. If what the administration is trying to do is to send an early, clear sharp signal to the Soviet Union of what will be new and distinctively different about its approach, El Salvador is surely handy, and probably safer for the short haul than, say, the Persian Gulf, or Poland, or Afghanistan.

The question is how sound it is for the long haul. And this has less to do with whether El Salvador and Vietnam are strictly analogous than whether, in its efforts not to duplicate Vietnam, the Reagan administration may not be ignoring one of Vietnam's more enduring lessons.

That lesson has to do with the wisdom of establishing make-or-break test cases on a global scale in local conflicts over which the United States can hope to exercise only limited control.