President Carter's choice for U.N. ambassador, Rep. Andrew Young (D-Ga.), said yesterday that the new administration intends to move forthrightly toward approval for Vietnam to join the United Nations. The Ford administration twice vetoed Vietnam's entry.

Young suggested, moreover, that Secretary of State Cyrus R. Vance will reopen talks with Vietnam within 90 days aimed at eventual diplomatic recognition.

Young also told the Senate Foreign Relations Committee - which voted unanimously to approve his nomination after three hours of hearings - that a negotiated settlement of the racial conflict in Rhodesia is still possible despite this week's breakdown in the British-led talks.

If he is confirmed by the Senate and sworn in by this weekend, which seems likely. Young is to fly to Tanzania early next week for talks with black African "front line" presidents.

An aide to the Rev. Martin Luther King Jr. during the civil rights battles of the 1960s, Young is the first black to be named as U.S. ambassador to the United Nations. He won resounding praise from committee members for this past record and current ideas before, during and after his testimony.

His appearance was immediately followed by a tribute from Rep. John Buchanan, a white Republican from Birmingham, Ala., who praised Young for his attack on white supremacy and segregation, calling him "one of the people most responsible for setting our beloved South free from the shackles of yesterday."

Rep. Larry McDonald (D-Ga.), a prominent member of the John Birch Society, and Alan Ogden, spokesman of the U.S. Labor Party, opposed the nomination. McDonald charged that Young had associated himself with Marxists and militants, while Ogden attacked Young for being a member of the Trilateral Commission.

Asked about admission of Vietnam to the United Nations, Young said he would "certainly hope" that the United States will now permit Vietnam's entry and said it is the intention of Vance and Carter to "move forthrightly" in this direction. He expressed hope that Vance will be in bilateral discussions with Vietnam within 60 to 90 days. Previous low level U.S.-Vietnamese talks in Paris made little progress.

The problem of a Vietnamese accounting for U.S. missing in action has been substantially reduced by a recent House committee report saying that only a limited accounting is possible. If this issue can be satisfactorily handled, Carter "has no objection whatsover to moving to some kind of diplomatic recognition and admission to the United Nations," Young said.

Despite Rhodesian Prime Minister Ian Smith's rejection of a British-sponsored plan for transition to black rule, Young held out hope for continuation of talks and an eventual settlement, largely because of the vital interest of South Africa in avoiding an expanding racial war in the region.

"Rhodesia can't survive without South Africa," Young said. "If South Africa says negotiate, they [the Rhodesians] have to negotiate . . . If South Africa's security is jeopardized, they're perfectly capable of pushing Smith toward a negotiated settlement."

Young said the United States should develop a "carrot-and-stick" approach to encourage peaceful change in southern Africa. He said "a very hard-line policy" might be necessary in dealing with South Africa, but that he would not give up entirely on that country, which contains some "good people."