Washington — As President Reagan battles for aid to El Salvador, one of his biggest tasks may be laying to rest the so-called Vietnam syndrome. This is the concern that somehow the United States is being drawn piecemeal into another protracted conflict that will end with American troops being sent to Central America.
At this writing, the President was expected in his major speech to a joint session of Congress on Wednesday to assert in the clearest possible terms that El Salvador is not Vietnam. No consideration has been given to sending American troops to Central America, according to administration officials.
By going before a special session of the Congress on a foreign policy issue - the first time this has been done by a president since Jimmy Carter's address on a new US-Soviet nuclear arms control treaty in 1979 - Mr. Reagan was taking a political gamble. It might prove to be the only way he can generate the necessary support for his request of $110 million in emergency aid for El Salvador.
But by giving El Salvador such high priority, the President might also be helping to turn Central America into an emotion-laden issue in the 1984 presidential election. In a sense, Reagan was upping the ante with Congress, implying that if El Salvador was lost to Marxist-led guerrillas, it would be the fault of Congress and not his administration.
At the same time, the administration has made some efforts to reach an accommodation with Congress and to respond to congressional doubts about US Central America policy. President Reagan has agreed to a request made by a key subcommittee chairman, Rep. Clarence D. Long (D) of Maryland, that a special envoy be appointed to help the Salvadorean government find a basis for dialogue with its opponents on the terms and conditions for elections. But the move does not appear to signal any fundamental change in the administration's skeptical attitude toward efforts to induce the government and guerrillas to negotiate.
Administration officials say that the President will name conservative Democrat and former senator, Richard Stone of Florida, as special envoy. The choice is not likely to please liberal Democrats, and it certainly does not please Congressman Long, who would have preferred a career diplomat for the job. State Department officials had proposed several career diplomats. At the top of their list was Francis J. McNeil, now ambassador to Costa Rica, but the advice of State Department officials appears to have been rejected. This was seen by some observers as one more indication that policy on El Salvador, or at least the public rhetoric, is now being shaped primarily by two officials: National security adviser William P. Clark and United Nations Ambassador Jeane J. Kirkpatrick.
The powerful Salvadorean Chamber of Commerce, meanwhile, condemned as ''offensive and humiliating'' the US plan to send a special envoy to help negotiate peace with El Salvador's leftist guerrillas.
Whatever the divisions within the administration on moderate vs. hard-line policy, there does seem to be one thing on which nearly all top officials are united: El Salvador is seen as a strategically important country. Its location close to the US, Mexico, the Panama Canal, and Caribbean sea lanes carrying 45 percent of the crude oil shipped to the United States gives it this distinction. Even in the Congress, where there are doubts about tiny El Salvador's value as a strategic domino, many senators and congressmen realize that its ''loss'' would have some spillover effect in the region, particularly in next-door Honduras. Whether the ripples would actually shake Panama or Mexico is a matter of debate. But President Reagan and his advisers are said to believe that the implications of El Salvador's possible fall need to be clarified for the American people.
''If the guerrillas won in El Salvador, it would definitely create some pressure on Costa Rica,'' said Robert S. Leiken, head of a project on Soviet involvement in Latin America at the Georgetown University Center for Strategic and International Studies. ''It would draw Guatemalan troops down to protect Guatemala's border with El Salvador, and thus make Guatemala more vulnerable to internal attack,'' says Mr. Leiken, who has been a frequent critic of the administration's Central America policy. ''It would push Panama in a more neutral direction. It would create some pressure on Honduras. But it would certainly not change the situation in Mexico very much.''
State Department specialists argue, however, that the impact would be more serious. Said one official: ''It would promote rampant accommodationism'' throughout the region.
It is the fear of such a phenomenon occurring which may provide the basis for a consensus of support for the Reagan aid request in the Congress.
''The Congress is offering resistance, but it is also signaling that it doesn't want to cut El Salvador off completely,'' said Leiken, who says he thinks that the Soviet Union is using its involvement in Nicaragua mainly as a means of ''bleeding'' and distracting the US and its allies. Leiken does not expect an all-out Soviet effort to gain a dominant position in Central America.
In his speech to Congress, the President was expected to touch on two incidents which he regards as evidence of outside intervention: Libyan planes waylaid carrying arms through Brazil to Nicaragua and an announcement that Costa Rican authorities had stopped a Panamanian freighter apparently carrying explosives to that same country.