Secretary of State Cyrus Vance leaves Washington today on missions to Africa and the Soviet Union that are plagued with uncertainties and which take place at the lowest point of the Carter administration's diplomatic prestige.

Usually a sense of optimism attends the foreign travels of a secretary of state, but on this trip, expectations are so low, and the criticism of the Carter administration's diplomacy is so intense, that any success that Vance achieves would be a boost for the beleaguered administration.

Vance's predecessor, Henry Kissinger, often publicly exaggerated the odds against accomplishing anything on his missions, so as to enhance the results he privately expected to achieve. The self-effacing Vance, whose trips abroad have been studded with obstacles, engages in no such gamesmanship.

There are misgivings inside the Carter administration about the wisdom of even venturing on the first leg of Vance's journey, the African portion. Officials at many levels regard it as an unwarranted risk of Vance's prestige for him to plunge into a new round of talks in the Rhodesian political morass less than two weeks after Vance's return from Africa with President Carter.

Although there is a widespread belief that Vance is returning to Africa because the president was drawn into that committment during his visit to Nigeria, a Vance associate insisted yesterday that this impression "is totally incorrect."

Vance "created this trip," said his associate. "He honestly believes that this is the 'last cast,' and he is determined to make the effort" to head off the threat of a civil war between black factions in Rhodesia.

It was announced yesterday that Vance and British Foreign Secretary David Owen intend to proceed with their plans to visit Salisburg, Rhodesia, on Sunday, despite the announced refusal of Rhodesia's interim government to join an "all-parties conference" to discuss the former British colony's political future.

This can leave Vance and Owen in a weak bargaining position with both sides: the transitional government of white Prime Minister Ian Smith and the three moderate black leaders who have joined him, and the leaders of the Black Patriotic Front conducting a guerrilla war against Rhodesia from neighboring countries.

Neverthless, a Vance associate said yesterday, the interim government "has not slammed any doors about continuing to discuss the subject."

Vance and Owen will meet first in Dar es Salaam, Tanzania, with Patriotic Front leaders Joshua Nkomo and Robert Mugabe, and presidents of the so-called front-line, black-ruled nations supporting the guerrilla war. These talks will be followed by discussions in Salisbury with members of the new Rhodesian multiracial executive council, Smith, Bishop Abel Muzorewa, the Rev. Ndabaningi Sithole and Chief Jeremiah Chirau.

The Rhodesian executive council yesterday named nine black ministers to serve with nine whites in a ministerial council during the transition to black majority rule, scheduled for Dec. 31. The militant guerrilla leaders have denounced this whole process as a sham. Under the Anglo-American plan for Rhodesia, Smith's white minority government would have been replaced at the outset of a transition period.

In addition to preparing the African portion of his journey, Secretary Vance met yesterday with President Carter and other senior national security advisers on negotiationg strategy for the strategic arms limitation talks (SALT).

The president, speaking to the American Society of Nespaper Editors yesterday, employed the more guarded language he has been using recently about the prospects for completing the long-delayed nuclear arms accord. He said, "I think we have a good prospect this year of having success" in these negotiations.

Vance told the same audienceon Monday that there is no expectation for concluding the SALT negotiations in his April 19-22 visit to Moscow.