For months, the Soviet Union has been talking and acting as if the United States had unilaterally turned against detente and the resignation this week of Cyrus Vance only serves to strengthen this official line.

President Carter's choice of Sen. Edmund Muskie (D-Maine) to succeed Vance has evoked little comment here yet. But the Soviets for a variety of reasons are unlikely to identify the political realities implied in the president's selection: that out of his own political weakness, Carter selected a man with a national image and that this may give the new secretary of state added influencing in a shaping American foreign policy.

It is inconvenient for the Soviets to see this interrelationship, for they have officially labeled Vance a detente martyr, victimized by national security adviser Zbigniew Brzezinski, a man the Soviets dislike and distrust.

The Communist Party paper Pravda yesterday offered a characteristic interpretation of Vance's demise, written by veteran columnist Yuri Zhukov.

"Thus ended a long internal strife in the U.S. administration, during which two trends were in constant conflict: the most aggressive one was led by Brzezinski, and the other by Vance and his proponents in the State Department who supported, although not always insistently enough, continuation of detente and peaceful settlement of international problems."

It is a commonplace of Soviet propaganda to call Moscow's military intervention in Afghanistan defensive and legal, while seeing the aborted American rescue attempt as a provocative act of international piracy. Soviet media have lately been describing the mission as the forerunner of a U.S.-backed military putsch to overthrow the Khomeini regime as they search to deflect world outrage at their moves against an Afghan Marxist they did not like.

Vance's resignation fits well into this scenario, increasing its propanganda value to the Soviets. Foreign diplomats here were appalled at the news Vance had departed, knowing, as one put it, "that the Russians will use it in a way that could ruin his honor and dignity for what he did."

Having Brzezinski at last at centerstage is a prospect that delights the Soviets for the same reasons. As long ago as the first months of the Carter administration, the Soviets had picked out Brzezinski as their bitterest adversary. As the president moved to increase the U.S. military spending levels, urged NATO to improve its preparedness, supported new nuclear missiles for West Europe, and finally delayed SALT II ratification after the Soviet Afghan intervention, the Soviets have realized relations were going to be far more difficult anyway.

Brzezinski and later Carter himself have been convenient scrapegoats. The Soviets simply do not accept the idea at any level that their own military buildup or successes in Angola, Ethiopia, and Vietnam could have anything to do with these U.S. acts.

But signs of new strains in Moscow's foreign relations were present here today at the traditional Red Square May Day parade. More than 16 countries, including most of NATO, refused to send senior envoys to the viewing stands along the Kremlin wall to protest the Afghan intervention. Soviet President Leonid Brezhnev, just returned from a three-week Crimean vacation, seemed oblivious to the snub as he waved from atop the Lenin mausoleum to the diplomatic corps gathered in the warm spring sun.

And the 90-minute nonmilitary Soviet parade made its own statements about Afghanistan, with two huge signs among the vast portraits of party leaders, declaring "Chinese-American [conspiracies] -- hands off Afghanistan," and "Stop the interference of imperialistic forces in Afghanistan."

In addition, for the first time in five years, there were no slogans referring to the 1976 party congress' "peace program," which Brezhnev is personally identified with. Nor was there any slogan referring to the 1975 Helsinki agreement on European security, a principal achievement of Brezhnev's detente strategy. Some analysts here have interpreted these omissions as signs of the much tougher Kremlin line that has emerged since Afghanistan.

In anticipating problems ahead, such as Brzezinski's ascendancy, the leadership is guided heavily by its own longevity in power. Andrei Gromyko, now 70, became foreign minister in 1957. Since then, there have been six American presidents, one Soviet change of leadership, two Soviet military invasions, one American war, two strategic arms limitation agreements, and two tense confrontations -- Berlin and Cuba -- that brought the rivals frighteningly close to war.

Along with all these crises and achievements, Gromyko has dealt with six secretaries of state. This is the kind of continuity from which the Kremlin draws so much of its power in international relations: it can wait for years for events to shape in its favor, whatever the worries of the moment, such as over Brzezinski or the eclipse of moderate State Department advisers such as Marshall Shulman.

Beyond that, it is suggested here, the Kremlin realizes that because of West German and U.S. elections this year, it would be very difficult under any circumstances to transact major business with the two most important Western powers.

With the Germans likely to join the U.S. Olympic boycott, and no sign the United States is interested in reconsidering its own economic sanctions, the Soviets may turn to their tentative "peace initiative" more actively in the next weeks to improve their world image. Gromyko may meet Muskie in Vienna later this month at the 25th anniversary of Austrian neutrality. If he does, he is sure to know exactly what he wants to say to his seventh American secretary of state.