A congenial visit here with Secretary of State Cyrus Vance by a moderate black African leader opens a sliver of hope that the Carter administration will reverse course and help an internal Rhodesian settlement long advocated by Prime Minister Ian Smith.

Vance conferred privately Nov. 15 with the Rev. Ndabaninghi Sithole, who returned to Rhodesia last summer after two years of exile. To insiders, Sithole expressed pleasant surprise that Vance did not seem wedded to the Soviet backed Patriotic Front as the future government of Zimbabwe (Rhodesia under black rule). Any U.S. nod toward Sithole takes on importance, for he is vital to the proposed peaceful transition to pro-Western black majority rule under one man, one-vote that Smith announced last week.

Whether the Carter administration really has embarked on this course remains to be seen, considering President Carter's refusal to see Sithole. But even a trial separation of the United States from its marriage to the Patriotic Front would be significant, for the misalliance certainly has promoted neither Western interests nor peace in Rhodesia.

Actually, Smith privately committed himself to one-man, one-vote during his 1975 talks with then-Secretary of State Henry Kissinger. When we visited Phodesia last June, it was taken for granted that Smith's public demand for a "qualified franchise" - a vote weighted for whites - was merely a bargaining point to be negotiated away.

At that point a Rhodesian settlement may have been closer to realization than anyone realized with South African Foreign Minister R.F. (Pik) Botha, a political moderate, deeply involved in secret diplomacy. But all hopes were dashed by the Aug. 5-6 state visit to Washington of President Julius Nyerere of Tanzania.

Nyerere returned to his capital of Dar-es-Salaam claiming Carter's agreement ("in writing") that the Smith regime's security forces would be disbanded and that the army of Zimbabwe would be dominated by Patriotic Front guerrillas. That alienated the South Africans, outraged Smith and blasted hopes for a solution.

Although the State Department officially denies any pledge "in writing" to Nyerere, there is little doubt that the Tanzanian leader was correct in substance (as later confirmed by the Anglo-American proposal on Rhodesia). In short, the Carter administration tied its Rhodesian policy to what Tanzania and the other so-called "frontline" African states wanted: a Patriotic Front government, which guarantees a Zimbabwe with no future for white Rhodesians, Western interests or black moderates.

But in recent weeks, the "frontline" states have fallen out among themselves. President Kenneth Kaunda of Zambia has been pushing to establish his client, Patriotic Front Leader Joshua Nkomo, as leader of Zimbabwe, postponing elections to some dim time in the future. Nkomo controls a Soviet supplied guerrilla army based in Zambia but has little popular support within Rhodesia. Nyerere, linked to Nkomo's rivals within the Patriotic Front, has vehemently opposed Kaunda's push.

At the same time, Smith was showing gains for an internal settlement. Although Sithole, since his return to Rhodesia, has been willing to negotiate, the other nationalist leader outside the Patriotic Front, Bishop Abel Muzorewa, (also back in Rhodesia), for months has refused to talk to Sithole, much less Smith. But Muzorewa's Lieutenant, James Chikerema, has recently given favorable signals - a breakthough for talks prompting Smith's statement last week.

Muzorewa remains the most popular fugure among Rhodesian blacks, though some key followers have defected to Sithole. Together, they probably command over 80 per cent of the country's blacks and an unknown number of black guerrilla fighters nominally loyal to the Patriotic Front.

Sithole is a legitimate Nationalist (frequently arrested and detained) who cannot be called Uncle Tom but who talks about the new Zimbabwe's indispensable need for Rhodesian whites, Western capital investment and free, multi-party government. He talks that way to blacks and whites in Rhodesia and to Cy Vance in Washington. Sithole left Vance's office encouraged to think there is no U.S. commitment to the Patriotic Front, which spurns whites, capitalism and demorcratic government.

But cautious insiders see only a marginal shift to the moderates by Vance. Although Sithole's importance to the Rhodesian situation was carefully explained to the White House staff, no time for a meeting could be found on President Carter's schedule.

Finally, there is the influence of Ambassador Andrew Young, whose ties to Nyerere, Kaunda and other "frontline" leaders are as emotional as his hostility to the Smith regime. Thus, the question to be resolved is the choice between Young's deep feelings on one side, and on the other need fora moderate, pro-Western biracial Zimbabwe.