The United States and Vietnam will resume talks in Paris on establishing complete diplomatic relations without delay and without preconditions, President Carter announced yesterday.

Carter said he "will respond immediately" to accept Vietnamese Premier Pham Van Dong's offer to resume the talks.The offer was brought back by the five-member commission Carter sent to Vietnam and Laos March 13 to ask about 2,456 Americans still missing after the Vietnam war, and to lay the groundwork for normalizing relations.

That commission delivered its report to Carter yesterday morning. Leonard Woodcock, head of the group, said the commission is satisfied there are no missing Americans left alive in either Vietnam or Laos. The Vietnamese said no Americans are being held against their will, Woodcock said.

A similar conclusion was reached in December by Rep. G. V. (Sonny) Montgomery (D-Mass.), chairman of a Select Committee on Missing Persons in Southeast Asia. Montgomery was also a member of Woodcock's commission.

Woodcock also said the Vietnamese told the commission they were not aware of any American deserters who had "joined the Vietnamese community."

In a brief appearance before reporters, Carter praised the commission, calling its mission "superb," and saying "every hope that we had for the mission has been realized."

The group was received "with great griendship," Carter said. "The Vietnamese have not tied together economic allocations of American funds with the MIA question. We believe that they have acted in good faith."

Vietnam promised to "set up a permanent study mechanism" to seek information about the potential whereabouts or identities of missing servicemen, the President said, and promised to cooperate in following any such information.

"They have also suggested that we reinitiate diplomatic discussions in Paris without delay to resolve other issues that might be an obstacle of peace between our two countries . . . and normalization of relationships between our two countries," Carter said.

"Normalization of relations" would mean that the two countries would exchange ambassadors and lift barriers on trade and travel.

The commission also brought back a letter from Premier Pham to the President. White House press secretary Jody Powell said it was a personal letter, and the contents would not be made public.

Carter said one of the 12 bodies delivered by the Vietnamese to the commission was not that of an American serviceman, and will be returned. "We have notified the Vietnamese government about the error and it was an honest mistake," he said.

Woodcock, president of the United Auto Workers, said identification procudures carried out in Honolulu indicated it was the body of a Vietnamese about 50 years old.

"We are absolutely conviced that this was a human error and simply underscores the enormity of the task of recovering those missing in action," Woodcock said. "I think . . . in World War II and in the Korean War, 22 per cent were finally not accounted for. In this war, the number is less than 4 1/2 per cent."

The United States first began preliminary talks with Hanoi about the possibility of beginning normal relations Nov. 12, evidently without much success. A week later the United States vetoed Vietnam's application for admission to the United Nations.

Afterward, the Ford administration reiterated its position that Vietnam must account fully for Americans missing in action as the first step toward normal relations.

Carter made that position a major campaign issue, calling Ford's refusal to send a mission to Hanoi to seek more information on those missing "one of the most embarrassing failures of the Ford administration."

In a March 20 interview with Associated Press correspondent Peter Arnett, who accompanied the commission throughout its five days in Vietnam and Laos, Woodcock said he told the Vietnamese "this was the best group they could ever get from America . . . I told them if they closed the door on us, then it might take 10 or 12 years before we were back."

In that same interview, Woodcock said the Vietnamese initially laid out tough demands for economic and construction aid.

Jerrold Schecter, press secretary for the National Security Council, said the administration does not know yet when the Paris talks will begin, or who will represent the United States.

The commission was less successful in its visit to Laos. "They indicated to us that they would set up an agency for the purpose of seeing what could be done relative to the recovery of those missing in action, "Woodcock said." . . . We came away with some hope which we think has been confirmed, but all of that lies in the future."

Laos was a last-minute addition to the trip. The commission was unsuccessful in attempts to get permission to visit Cambodia as well.