Remember Cyrus Vance, the former secretary of state, deputy secretary of defense, secretary of the army; the peace negotiator and trouble-shooter in Vietnam, Cyprus, Korea, the Detroit race riots; and now the quintessential Wall Street lawyer?

Turns out he's a dupe of the Communists. It was in the papers, straight from the Reagan high command. Not for attribution, of course; that's not how mud is slung in this town. But in leaks, with an altogether authentic ring, it was said that event he president was shocked by the way Vance provided grist for Soviet propaganda mills by questioning (in a recent interview on "Meet the Press") the administration's sincerity on arms controls, as well as the way it handled the new arms-sales-to-China policy.

That this is hogwash hardly needs saying. Vance reported his impressions, based on what the Soviets told him on a recent trip to Moscow, of their doubts that the United States is serius about arms control. He did express his own doubts about whether the administration has "a policy yet with respect to arms control." But he did not do so nearly as categorically as did Eugene V. Rostow, the president's very own nominee to run the arms control agency, at his Senate confirmation hearings.

So much for Vance giving aid and comfort to the Soviets. That the Russians talk in propagandistic, self-serving, deceptive ways to visiting Americans, official as well as unofficial, is no reason not to report it. That Haig's clumsy handling of the China arms matter surprised even members of the administration is no secret.

The point is not the mud (Cyrus Vance's faithful public service speaks for itself), but the mud-slinging. It says quite a lot about the continuing incapacity of this administration, six months into its first term, to deal with the natural vicissitudes -- the occupational hazards -- of managing almost every aspect of foreign policy.

The Vance case is a minor bit of meanmindedness. But it is of a piece with the far more consequential and equally furtive White House number now being done on Secretary of State Alexander Haig.

There's no way to prove it beyond a doubt (no one steps forward to take credit in these matters), but by a process of simple elimination you have to figure that both are the work of the White House inner ring. That includes the old-time political intimates (White House aides Ed Meese, Jim Baker and Mike Deaver) with an assist from Richard Allen, the president's adviser for national security.

And both incidents reflect the same severe shortage, in the handling of foreign policy, of precisely the qualities that have distinguished the administration's handling of domestic (chiefly economic) affairs: the Mr. Nice Guy approach, coupled with sureness of purpose, competence, tough-mindedness and a reasonably decent respect for dissent.

Whether we are talking about the overwrought reaction to the criticisms of Cyrus Vance or the White House vendetta with Al Haig, the common denominators are of quite the opposite sort. There is a sense of lack of discipline and extreme defensiveness, of incoherence and small-mindedness, all around.

It is true that Haig has brought a lot of his troubles on himself by his reach for authority, his insistence on leaving his mark on everything, his sometimes rattled, sometimes overly contentious, attitude. All this has invited the leaks and whispers of a "Haig problem": he is not a team player; he has "alienated" the president; is it "medical"?

Whether Haig has overreached or the White House powers-that-be are being overprotective -- of the president or themselves -- is less important than the effect this is having in a town where rumor mills are almost the only light industry.

At best, the effect is embarrassment for the president and his secretary of state. At worst, it is beginning to be destructive -- of Haig's effectiveness and of respect for not only the president but the performance of the U.S. government.

When the rumor-peddlers are making book on how soon Haig may be replaced (and by whom), foreign diplomats and dignataries are, well, confused. One veteran ambassador from a close ally already is brooding out loud about the problem of knowing where the power lies: "Who should I be talking to? Who has the last word?"

In short, there is something uncommonly rancid about the atmosphere. The high incidence of mud-slinging is but one measure. White House "officials," it is said, are aware of it, and of its potential for harm to American diplomacy. But they are not sure what to do about it.

Inasmuch as they are part of the problem, that's understandable. The solution, when it comes, will have to come from the president.