YOU CAN OVERDO this thing about the president's failure to attend the funeral of President Tito in Yugoslavia, and we think a lot of people are doing just that. The weakest part of the argument seems to us that which holds that in staying home Mr. Carter forfieted a rare opportunity to use a mellow and timely occasion to resume a much-needed "dialogue" with Moscow, since Leonid Brezhnev, among numerous other foreign moguls, was there. But surely the overriding foreign policy requirement of the Carter presicency right now is not to resume a "dialogue" with Moscow. It is to determine what the nature and purpose of any such "dialogue" ought to be.

Somewhat later than a lot of other Americans, Mr. Carter decided that his earlier perception of the Soviets' approach to power was no longer adequate. But he has yet to disclose a more serviceable new one. Having abandoned one secretary of state, he also would have been rather rash to engage in new direction-setting with Moscow, or Moscow's headman, without first consulting and coordinating with his new secretary. And in any case, a convenient occasion was at hand, if one were needed, in Vienna next week for an exchange with the Russians at the foreign-minister level.

The more plausible and damaging complaint was that the president, in staying away, may have sent absolutely the wrong signal to both the Russians and the Yugoslavs -- i.e., that this country in general and its current government in particular were, if not indifferent to, at least not especially interested in the Yugoslavian present and future. That is wrong -- wrong as a signal and as an idea, and it will be too bad if the president's absence has been received that way. It shouldn't have been. After all, no one could argue, at least not with a straight face, that the ostentatiously mournful presence of Mr. Brezhnev ment, or was seen by the Yugoslavs to mean, that the Russians took a benevolent and solicitous interest in Yugoslavian well-being.

The fact is that no concievable affirmation of Mr. Carter's concern could be nearly as valuable to the Yugoslavs as a demonstration by him from this point on that he is in charge of American policy and that he has a serious and sustainable idea of how to deal with Soviet Power. The trouble, of course, is that the thing isn't always seen this way, and once it was considered and complained about that Jimmy Carter had shown by staying home that he didn't much care -- well, the complaint became in its way self-fulfilling. Probably he should have foreseen this and taken some steps to head it off, that is, done the diplomatic work to make his decision not to go less vulnerable to misinterpretation and the charges filling the air now. If he wasn't going to go, in other words, he should have taken some kind of strong action to neutralize and manage the effect this would have.

All this would have been nothing more than the simple exercise of diplomacy. And speaking of diplomacy, it occurs to us that the president could use a lot more of it in his public remarks on the Secretary of State, Cyrus Vance, who has just departed. Mr. Carter -- any critical or malevolent intention has quickly been denied -- was very clumsy and graceless in his remarks comparing Messrs. Vance and Muskie in Mr. Vance lacked all sorts of virtues he looked forward to see in the Muskie stewardship at State. But if Mr. Vance was doing something other than the president's bidding and following a role other than that the president had carved out for him all these years, it will be quite a surprise to all concerned. If anything, it seems to us, he was probably too content for too long to accept the limits on his authority.

To suggest, even indirectly, now that Mr. Vance somehow lacked "statemanship" or leaderly size seems frankly petty and uncertain and just the opposite of that particular strength and self-confidence the president and the government he speaks for need most to show now. It is, as another signal, all wrong.