If President Reagan had merely forgotten Vietnam, his Central American disaster-in-the-making might be a little easier to understand. He hasn't forgotten. He simply hasn't drawn--he's assiduously avoided drawing--the obvious lessons from what he remembers. He knows that a lot of us are afraid of where his policies are taking us, because we remember Vietnam, so he seeks to reassure us that there is "no comparison with Vietnam." Central America isn't Southeast Asia. El Salvador and Nicaragua are not Vietnam.

Wrong. Except for some trivial details, we are in Vietnam. It is, if anything, worse than Vietnam because we experienced Vietnam. The president knows, or ought to, that "our side," while maybe not "the most bloodthirsty (regime) in our hemisphere, perhaps in the world," as Jimmy Carter put it recently, can hardly qualify as the good guys in some conflict between good and evil. There aren't any good guys, which is reason enough for us not to be shoving such huge piles of diplomatic and military chips into that losing pot.

He knows, or ought to, that U.S.-trained Salvadoran troops are unlikely to win anything that is, from our point of view, worth winning, and that the increasing presence of U.S. advisers promises only to draw us into the actual fighting. He knows, or ought to, that all the talk about helpless giants and flagging American will is rhetoric aimed at getting us more deeply involved than the American people want to be, because we remember Vietnam. He knows, or ought to, that a little more money, a little more advice and a few more U.S.-supplied arms aren't going to make the crucial difference.

And yet Reagan doesn't seem to understand any of these things, and may even be courting the chance to show how a real tough-guy president would have salvaged our honor in Vietnam. El Salvador isn't Vietnam? Just look at some of the headlines in this newspaper in the last several weeks: "Salvadoran Victory Predicted," "El Salvador Strives to Retain Trained Troops," "Weighing the 'Saving' of Latin America."

Not Vietnam? Reflect on the sorry history of our involvement in that place that most of us still have trouble locating on a map: first, token help to prop up a regime threatened by communist-inspired insurgents; then the escalation of our support in money and arms, each time on the supposition that the next infusion will enable the distrusted government to turn the corner; then the widening of our role in the conflict to include our client's neighbors, and the escalation of the rhetoric that insists our vital interests--even our physical security--are in the balance.

Not that we intend to commit American troops to the struggle. Our goal is peace. The thousands of Marines being sent to Honduras will only be participating in "military exercises," the point of which is to intimidate the bad guys in Nicaragua into behaving themselves. And what if they aren't intimidated, because they cannot afford to let themselves be intimidated?

Well, the problem is that our side would have won long ago except for the steady influx, through Nicaragua, of communist arms. The obvious solution is to cut off that arms supply by quarantining the area--you know, cut the Ho Chi Minh Trail. Does anyone outside the administration seriously believe it will work? Can anyone doubt that the most likely outcome is an eventual leftist victory that will expand the communist foothold in the region, and that the United States will end up less influential, more roundly hated, and more clearly than ever the bad guys of Latin America? And that doesn't even take into account the political fallout at home, when American GIs begin to die.

Not Vietnam? Just you wait.