To suggest even some Vietnam "echoes" in El Salvador, as I did in this space last week, is to invite an instant argument that for all sorts of reasons--geographic, demographic, the altogether different order of magnitude--the two struggles are not analogous. Did not Ronald Reagan, in his April address to the joint session of Congress, clearly say, "To those who invoke the memory of Vietnam: there is no thought of sending combat troops to Central America"?

That he did. But even then, alarm bells ring: in the denial of intent (carefully crafted, according to reliable informants) that falls just short of a firm commitment; in the apparent limit on what the United States will do to control the outcome of a conflict whose capacity for dire consequences to U.S. interests is proclaimed to be almost limitless.

And the ringing doesn't end there. For a grim specific, consider the cold-blooded killing of the first U.S. military adviser, Lt. Cmdr. Albert A. Shaufleberger. The official administration reaction to it--and shock and sorrow--was something everybody shared. Harder to comprehend was the administration's outrage and surprise that this conflict could take such an ugly turn--once again a Vietnam echo unheard.

"These are not freedom fighters," the president exclaimed in one of those shouting matches with newsmen over the roar of a waiting helicopter. He and other high officials saw it as a "murder" by "terrorists," a "cowardly" act. A Pentagon spokesman saw it as the work of "a madman."

The awful irony is that Shaufleberger knew the method in the madness. In an interview with Lydia Chavez of The New York Times just a few days before he was killed, he demonstrated precisely the sort of hardheaded and sophisticated understanding of the nature of revolutionary struggle (not just in Vietnam) that seems to be missing in the Reagan high command. It was quite possible, he warned, that U.S. military advisers would be singled out as targets by rebel forces. That struck him as a logical retaliatory tactic by heavily outnumbered guerrillas confronting a toughening U.S. policy and a sharp division in U.S. opinion on its wisdom.

"They haven't targeted Americans because things are going so well," Shaufleberger said. "But if President Reagan is successful in getting aid they are going to get nasty." He did not seem to see this as a sign of cowardice--just nastiness. "Shooting a soldier in the line of duty," he told his interviewer, "is a lot less risky than shooting a female counselor officer."

Shaufleberger knew his enemy at first hand. He was trained in counterinsurgency. So it is not unreasonable to suspect that he was also schooled in the record of Vietnam in the fall of 1964 and the spring of 1965, when Hanoi embarked on a campaign of calculated attacks on the encampments of U.S. military advisers in South Vietnam.

The first was in October at a place called Bien Hoa, where the United States had six B57 bombers parked on a runway. They were there as a token of U.S. readiness to repeat the August raids on the North in response to the incident involving two U.S. destroyers in the Tonkin Gulf. A surprise attack by Viet Cong forces killed five Americans, wounded 76 and destroyed the B57s at Bien Hoa.

A second "terrorist" act on Christmas Eve, 1964, on an American officers' quarters in Saigon, killed or wounded 45 Americans. On Feb. 7, 1965, a third American installation at Pleiku was attacked; seven American soldiers died and 109 were wounded.

By that time, the Johnson administration had solid intelligence, including intercepted enemy radio transmissions, indicating that this was no random pattern, but a pinpointed effort to turn the American public against the Vietnam effort. The results were just the opposite: the beginning of retaliatory American air raids against the North, followed in March by the landing of the first U.S. combat forces, ostensibly to protect the American air bases in the South. So, when a Salvadoran rebel group claims credit in a radio broadcast for Shaufleberger's death, and promises to make El Salvador a "tomb" for Americans, it is not hard for students of insurgency technique and experts on the Central American scene to find, in miniature, some echo of that 1964- 1965 Vietnam experience. "What this says is that the rebels have penetrated the capital of the country and have a capability to make good on their threats," one knowledgeable authority contends. "This was a professional operation, in broad daylight, on a crowded campus."

The concern of these observers (and of Shaufleberger) may prove to be misplaced. But the response of the Reagan administration to the first such incident raises deep questions about its ability to understand, let alone to deal with, a tactic of "targeting Americans"--if that's what's in store.