At least one department of the U.S. government seems to have learned a lesson from Vietnam. It is, praise be, the Pentagon, which is displaying the most uncharacteristic restraint in the face of temptation in El Salvador.

Secretary of State Alexander M. Haig Jr. has been decanting inflammatory rhetoric about military action from the first days of the administration. The thought of refighting Vietnam, of showing the Soviets who's boss in this hemisphere, had powerful appeal for him. His aversion to guerrillas either singly or in groups is exceptionally well known.

He went on about "international terrorism," about "going to the source," about the East-West nature of the conflict. Lately, and more ominously, the general in charge of our diplomacy has been hinting at a blockade of Cuba and Nicaragua.

"I've never seen this smoke without a fire," says an anxious congressman.

During all of this, Secretary of Defense Caspar W. Weinberger, who would have to run the "splendid little war" that Haig has in mind, has been curiously silent. The hard-liner in the Pentagon, who ceaselessly warns us of Soviet strength and guile, seems oblivious to the threat in the Caribbean.

For this, we must thank the generals. They fought one jungle war, in a small country with open borders, and they are not keen for another one. Besides, on the eve of receiving the largest allowance ever proposed for the Defense Department, they do not want the bad publicity that such murky adventures bring.

In his news conference, President Reagan indicated that he realizes he must pay lip service to the bitter Vietnam remembrance. Asked about the "contingency plans," Reagan said firmly: "We have no plans for putting Americans in combat any place in the world."

But he was vague about El Salvador, disputing claims of "military stalemate" and repeating the Haig line that the whole mess is simply an example of "exported revolution."

The reason that the plans do not get off the drawing board is that Weinberger is not keen. It isn't that he doesn't subscribe to Haig's inflammatory rhetoric. It's just that he thinks it would be awfully difficult to fight a war.

Moreover, his recent conversations in Europe, where he encountered heavy weather while peddling neutron bombs and new nuclear weapons, have persuaded him that intervention in El Salvador could cost too much--it could vindicate the Soviets retroactively in Afghanistan and prospectively in Poland.

While Reagan was speaking Tuesday, Salvadoran Defense Minister Jose Guillermo Garcia was on Capitol Hill, speaking to Rep. Mary Rose Oakar (D-Ohio), who keeps demanding to know who killed the four American missionaries in El Salvador last December. During his recent visit here, Jose Napoleon Duarte, the hapless civilian president of the Salvadoran junta, repeatedly was asked why justice had not been done in that case.

It was borne in on him that the Salvadoran government's cavalier conduct of the case had eroded all confidence in the junta's good will and effectiveness.

Garcia brought Oakar a 100-page volume, in Spanish, of evidence collected against six Salvadoran soldiers imprisoned but not charged in the slayings. He also brought material explaining Salvadoran law.

Garcia also called on Rep. Michael D. Barnes (D-Md.), chairman of the House subcommittee on inter-American affairs, who urged on his visitor the importance of negotiating with the leftist forces.

The Senate recently voted unanimously to require the president to name a special envoy to El Salvador to institute talks with all parties and El Salvador's Central American neighbors. Sen. Mark O. Hatfield (R-Ore.), author of the amendment,, received angry calls from both Haig and Thomas O. Enders, assistant secretary of state for inter-American affairs. Hatfield heard nothing from the Defense Department.

The administration is adamantly against negotiations.

The bodies continue to pile up, and the atrocities go on.

Official opinion may be divided; public opinion is not. Opposition to a "demonstration war" on this continent is as strong as opposition stirred by Haig's recent mention of a "demonstration nuclear blast" in Europe.

Weinberger went to Arlington Cemetery yesterday for Veterans Day observances. All around him, Vietnam was being refought verbally--acrimonious arguments about the memorial planned for the Mall, about television shows commemorating the veterans, about Agent Orange and rap centers and national ingratitude.

Weinberger made the hawk's familiar pledge: "Never again will we ask young men and women to serve in a war we do not intend to win."

The doves read it as further affirmation of his antipathy toward military action in the Caribbean. To them, he was saying: "Don't tangle with guerrillas." That, of course, is something the secretary of state has yet to learn.