In a final diplomatic effort the outgoing Ford administration and the incoming Carter administration joined in trying to prevent collapse of the Rhodesian peace talks.

A collapse of the current, British-led drive to keep the Rhodesia talks afloat presents a danger of "progressive deterioration" of peace prospects across southern Africa, Carter administration sources acknowledge yesterday.

Secretary of State-designate Cyrus R. Vance and other Carter administration officials conferred with Secretary of State Henry A. Kissinger Tuesday night on the Rhodesian dilemma.

Diplomatic sources said Kissinger and Vance both have strongly urged South Africa's Prime Minister John Vorster to use his influence on Rhodesian Prime Minister Ian Smith to keep the diplomatic track open. South Africa is said to have reported yesterday that it has carried out the request.

British peace envoy Ivor Richard and Vorster met yesterday in Cape Town where Richard said after a two-hour meeting, "It was a very useful conversation."

Richard is chairman of the Geneva conference on Rhodesia, which is deadlocked and out of session in a dispute over control of Rhodesia's army and police during a transition from white minority control to black majority rule.

South Africa, Rhodesia's main supporter, controls landlocked Rhodesia's access to the outside world. Richard has spent the last two weeks in Africa exploring new British proposals. They replace the original American-British plan that Kissinger presented last September for majority rule in Rhodesia.

Richard plans talks with Smith in Salisbury on Friday, following further discussions with black nationalist leaders. Richard also intends to return to Cape Town to see Vorster again before the end of the month, British sources said last night.

However, in Salisbury yesterday, Rhodesian government sources said that from what little they know of Richard's new proposals, they are unacceptable to Rhodesia's government.

A Rhodesian source quoted by United Press International said: "It seems the proposals call for a resident British high commissioner with a casting vote in the proposed multiracial council of ministers or cabinet," which would be two-thirds black and one-third white.

"This means," the source said, "the high commissioner will have a commanding position and this is not likely to be accepted."

On the eve of Richard's latest visit to Salisbury, a Rhodesian minister reemphasized the government's desire for a settlement with the "moderate African opinion" among the nation's 6 million blacks, who are now ruled by 270,000 whites. Foreign Affairs Minister R. K. Van der Byl in an interview talked prospects for a settlement with Bishop Abel Muzorewa's United African National Council, and tribal chiefs in the Zimbabwe United People's Organization.

Africa's black front-line nations in the Rhodesian guerrilla war have given their full support to the rival Patriotic Front, led by joshua Nkomo and Robert Mugabe.

In another diplomatic area, the first open sign of disagreement appeared yesterday between the outgoing Ford administration and the incoming administration.

On Tuesday, President Ford resubmitted to Congress a proposed new, four-year U.S.-Turkish security cooperation accord, to supply Turkey with $1 billion in loans, grants and credits. Congress failed to act last year on the same request which is intended to keep open U.S. military installations in Turkey, closed in the Greek-Turkish dispute over Cyprus.

An aid to Vance complained last night that the Ford administration previously had said it would not submit the new request because the Carter administration was conducting its own review of the matter. "We hope," the aide said, "that no action will be taken by the Congress until this review is completed."