If you're confused about where the Reagan administration is heading in El Salvador or just how it proposes to resist the threat of communist encroachment in Central America, it's probably because you've been paying attention.

Take the question of sending American troops to El Salvador, posed a half- dozen different ways at the president's press conference this week. Could he envisage that happening under any circumstance? "Maybe if they dropped a bomb on the White House," he replied. But moments later he volunteered a garbled version of the furtive progression from a handful of military advisers in Vietnam to the first landing of American combat units in a way that could only feed the worst fears of congressional critics of his policy.

"There are no current plans" for using American forces, is the way Secretary of State Al Haig puts it. "On the other hand," he adds by way of clarification, "the sterility of drawing lines around America's potential options constitutes the promulgation of roadways for those who are seeking to move against America's vital interests."

The assistant secretary of state for the region, Thomas O. Enders, told a congressional hearing the other day, "Our goal is not a military victory," implying that out of next month's legislative elections would evolve a political process, ultimate reform of the more repressive elements in the military, a popularly elected government and some measure of stability. Earlier, the U.S. ambassador had taken a dimmer view: that the elections might prove meaningless if the leftists refuse to participate, in which case "you can be forced to continue the fight."

Stay tuned, we are advised, for a bold new administration initiative in concert with Latin allies and combining both covert "paramilitary" action and some form of collective economic buttressing to get at the root of communist-exploited insurgency. The details may be laid out in a special presidential address.

Meanwhile, definitely ruled in are enormous increases in military aid for El Salvador in the 1983 budget, including more--and more sophisticated--combat aircraft. There will be much more economic aid as well. The totals, in the hundreds of millions, will make tiny El Salvador one of this country's half-dozen largest aid recipients, with no fixed ceilings for the future.

And that, of course, is the heart of the administration's predicament in El Salvador. It is not just that "our side" is only nominally a Christian Democratic regime, held in place by a repressive military cabal beholden to a rapacious oligarchy; that is not new to this country's Latin American experience. And it is not just the bitter controversy over the ease with which the administration almost routinely provided the necessary certification of progress on reform of the murderous practices of the Salvadoran military--a certification mandated by Congress as a condition for continued American aid.

It is all of this added to the specter of yet another open-ended "limited" war-- "another Vietnam." It is not necessary to accept the analogy to understand how it plays acutely on public and congressional sensitivities. "Body count" is back in the language in El Salvador, this time as a measure of non-combatant deaths in "human rights" reports. An inquiring senator presses an administration witness for some sign of "light at the end of the tunnel."

Granted that Vietnam is too freighted with too many different meanings and memories, of how the war was waged and the consequences of its loss, to hold up for long as an analogy to El Salvador. The scale doesn't fit, and still less the geography.

But creeping, insidious involvement, accelerating escalation, misplaced optimism, chronic underestimates of enemy capabilities, a calculated ambiguity about future intentions and an uncalculated confusion of purpose--all these elements of Vietnam can already be found to some degree in the record of the American experience in El Salvador.

All figure to some degree, perhaps not always consciously, in the intensity of congressional questioning of what gives every evidence of being a deepening and widening American commitment in El Salvador and Central America.

This questioning, to be sure, falls well short of a congressional revolt. But so, of course, did the first show of congressional dissent on the Vietnam War; it was only as the struggle there dragged on without demonstrable progress that congressional opposition and public protest began slowly to make themselves felt.

In El Salvador, I suspect, disenchantment could set in much more swiftly, and with decisive impact, if only because there are so many ways--a coup from the far right, a string of rebel successes or a wave of excesses by security forces--that the American effort could be undercut.