WE ARE NOT familiar with the particular CIA secret plan to drag America surreptitiously into the Vietnam War, to which a questioner referred at President Reagan's press conference yesterday. The principal secret plan we remember in connection with the Vietnam War was the one candidate Nixon was said to have for ending it. That was in the fall of 1968. By the time the war did end seven years later, one could see why the plan had been kept secret. But whether there was or wasn't some specific CIA secret plan of the kind mentioned yesterday and whether it did or didn't resemble anything that is or isn't being contemplated for Central America now, we do wish the president had kept his extraordinary answer to the question secret. It rambled. It tripped. It did no one any good. If we'd had access to the fabled rubber stamp, we think we'd have stamped it classified.

This is an unfamiliar and uncomfortable position for us First Amendment junkies, but the fact is that we think Mr. Reagan is talking too much at these press conferences or, if not too much, then too loosely and too lightly, almost too blithely. Even granting that Americans have a great deal of disagreement among themselves concerning the facts of our involvement in Vietnam, Mr. Reagan's "facts" were uncommonly unusual--which is to say bollixed. He could have said merely that he recalled no such plan concerning Vietnam, but that if there was one, it bore no relation to anything being contemplated now or that he had no comment or . . . almost anything but what he said.

The mixed-up history of the war, which he provided, put us in mind of the president's answer to a question about the Supreme Court's Weber decision at a press conference a couple of months ago. It was clear then (he said as much) that Mr. Reagan was winging it; and as a result he and his assistants had to do a lot of backtracking and explaining and regretting when it was over. Presumably that will now happen with his garbles on U.S. involvement in Vietnam.

Undeniably there is something engaging about a man in that particular press-conference, lion's-den situation, earnestly and affably and unself-consciously volunteering his various reflections in an attempt to argue or explain. But the result undermines Mr. Reagan's own credibility something awful. It calls into question the underlying basis of hundreds of other decisions he has made. Improvising under that pressure and under that barrage of questions is high-risk stuff. The president should stick to the script.