Proceed to the blackboard, the Reagan administration regularly tells its critics, and write 100 times: El Salvador is NOT Vietnam.

Consider it done. They speak Spanish in El Salvador. It is 1,000 miles from Key West, caught up in a chain of contiguous nations that leads to the Rio Grande. Its cramped quarters could not accommodate 10,000 U.S. troops, let alone Vietnam's half-million, even if any long-term gain could be realized by having U.S. combat forces chase insurgents or garrison towns that the Salvadoran army itself can't hold against rebel hit-and-run takeovers. This time, it may even be safe to believe the government when it says that an increase in U.S. military advisers, and some expansion of their activities, will not evolve into an actual combat role.

Even so, there remains a chilling resemblance, not between the exact nature or likely dimension of El Salvador and Vietnam, but between the mindset, the strategic concept and the language of the policy- makers then and now.

When the president seeks to reassure us that there are strict limits on the level and character of American military involvement, while putting almost no limit on the threat to our national security--and in the same breath insists "there is no parallel whatsoever with Vietnam"--he seems to be innocent of any sense of the Vietnam echo in what he is saying.

He seems unaware of the extent to which the administration's policy, as presented in recent days in a sudden outburst of public statements and White House leaks, is Early Vietnam: a little more of this, a little more of that, in response to our side's losses and the other side's gains, all carefully calibrated to make protracted conflict politically tolerable at home while at the same time convincing to the adversary.

It is not hard to figure out what brought some of this on. It's foreign aid appropriations time, and the administration appears to be seeking a giant increase in aid to El Salvador. It wants the $60 million in military aid (out of a requested $80 million) that got lost in the shuffle in last year's lame-duck session of Congress, plus roughly the same total in the coming fiscal year. And now there's talk of an exra $50 million in supplemental "emergency" funds. That's a high price to pay for a war effort that has nothing to show for itself after two years of heavy American support.

So there is predictable resistance from Congress. Equally predictable, there is an administration counter-offensive. The "domino theory" is back in vogue, and U.N. ambassador Jeane Kirkpatrick is back from a whirlwind on-the-spot inspection of the "dominoes" with some pretty hairy analyses of the communist menace.

Never mind that Central American government leaders are military-oriented and can be counted on to recommend more military assistance; being a "domino" isn't all bad if you get hazardous-duty pay. From Kirkpatrick, moreover, you don't have to listen to lectures about human rights and social reforms.

The administration is not dealing only in sticks. A carrot of sorts has been dangled in the prospect of free elections in December (four months earlier than expected), thanks to an intense diplomatic effort by the United States designed to look like the Salvadoran government's own idea.

Kirkpatrick, who also firmly rejects any Vietnam parallel, has come up with a nobler analogue: the Marshall Plan. She would have the United States take the initiative in a regional economic aid program for Central America. The idea is hard to fault, assuming that the Reagan administration is prepared to make the kind of fight that would be necessary to extract from Congress anything as grand as a "Marshall Plan" for the Caribbean area.

But the dominant administration theme is hard-nosed and largely military in its emphasis. It emanates from the White House--and the president himself. There is no reason to believe he sees the Central American problem today any differently from the way he saw it in the course of an interview on NBC in September 1980: "It's time the people of the United States realized that under the domino theory, we are the last domino."

It was Lyndon B. Johnson in the late 1960s, if memory serves, who tracked the Vietnam "dominoes" from Saigon to Hawaii and on to San Francisco with local stops along the way in Thailand, Indonesia, the Philippines. The more things change . . . .