Cyrus R. Vance, in a Harvard University address that serves as a valedictory for his tenure as secretary of state, appealed yesterday for ratification of the SALT II treaty before the end of this year to head off a dangerous nuclear arms race in the 1980s.

Vance, who devoted much of his time and energy in office to negotiations with the Soviet Union on the strategic arms limitation treaty, then the Carter administration's top foreign policy priority, said the treaty "stands at the very heart of a sensible and far-seeing American foreign policy."

Despite the Soviet invasion of Afghanistan and the presidential election in this country -- both of which he acknowledged as formidable barriers to early ratification of the stalled treaty -- "if we fail to act, we will someday ask ourselves why we were blinded by considerations of the moment and lost a vital, long-term opportunity," Vance declared.

Although he did not spell out this view, Vance is known to believe that, because of the timing of some of its provisions, the treaty probably will have to be renegotiated or abandoned if it is not ratified this year. And renegotiation, he fears, would be extremely difficult in the present climate.

The chances for Senate ratification of SALT II before the November elections are considered remote. Senate Democratic Leader Robert C. Byrd (W.Va.) said after being informed of Vance's remarks that the atmosphere created by the Soviet invasion of Afghanistan "has not changed" to improve the prospects for SALT II. Action immediately after the November election is considered doubtful, at best.

Speaking of the SALT imperative, Vance said "it is far too easy, in an election year, to let what may seem smart politics produce bad policies." At another point in the speech he said, "It is time to set and stick to basic goals. Neither we nor the world can afford an American foreign policy which is hostage to the emotions of the moment."

Those remarks, which could be taken as criticism of President Carter's current policies, were as far as the ever-loyal Vance would go to separate himself from the administration from which he resigned six weeks ago in disagreement over policy.

The immediate cause of Vance's resignation was Carter's decision to use military force in an attempt to free the American hostages being held in Tehran. But Vance had been losing battles and losing favor on a range of issues in the administration for many months before that.

The commencement speech to the Harvard graduating class -- Vance's only public statement since leaving the State Department -- contained no hint of retort to Carter's remarks in Philadelphia May 9 implying that Vance's successor, Edmund S. Muskie, would do a better job on several fronts. Carter is reported to have initiated an exchange of personal letters with Vance after the impromptu and highly publicized comments, in an effort to make amends.

In keeping with his tight-lipped practices in office, Vance did not take the occasion to illuminate any of the internal disputes that snapped his effectiveness as secretary of state.

In an oblique reference to one longstanding dispute, Vance went out of his way deep in the prepared text of his speech to call for U.S. diplomatic recognition of ANGOLA.he had long favored working with Angola despite the government's leftist nature and Soviet backing, but this policy was often opposed by those who saw conflicts in Africa in geopolitical and superpower terms.

Most of the address, which was written with the help of several former aides, was Vance's overview of the problems, pitfalls and priorities for the United States in the world of the 1980s, with stops along the way for potshots at the "self-indulgent nonsense" of "simplistic solutions and go-it-alone solutions" resulting from emotionalism or ignorance.

In Vance's rendition the dangerous fallacies -- "illusions [that] must be exploded before our nation can chart a coherent and determined course in foreign policy" -- are these:

The idea that "a single strategy" or "grand design" will yield the answers for U.S. policy in a complicated and pluralistic world.

A "fear of negotiation" which assumes that the United States will come out second best in any international bargain. "This fallacy was at work in the emotion underlying the opposition to the Panama Canal and SALT II treaties," Vance said.

The "myth" of incompatability between pursuit of U.S. interests and pursuit of human rights and other foreign policy "values." Vance said it is in U.S. interest to support "constructive change" -- as in the Dominican Republic and Central America -- to combat more radical or repressive outcomes.

"The dangerous fallacy of the military solution to nonmilitary problems," which is acute in times of frustration or seemingly tedious negotiations. Use of military force, Vance said, "is not, and should not be, a desirable American policy response to the internal politics of other nations." d

The "obsolete idea" that the United States can dominate the Soviet Union and "have the power to order the world just the way we want it to be." Vance said the Soviet position of "essential equivalence" is a fact, and "it is naive to believe that the Russians will play by our rules, any more than we will accept theirs."

Vance declared that "positive incentives" as well as military deterence are needed for the management of the increasingly difficult relationship with the Soviets. Such incentives have been put aside, at least for the present, by the Carter administration in the reaction to Afghanistan.