The United States will support Vietnam's admission to the United Nations in a Security Council recommendation that may come today as a sign of the Carter administration's desire to "put the past behind us," administration officials said yesterday.

After North Vietnam's conquest of South Vietnam in 1975, the United States repeatedly veoted U.N. membership for the unified Vietnam, primarily on grounds that the Hanoi government was failing to account for American servicemen missing in action.

At a meeting in Paris last May with Vietnamese diplomats, the United States said Hanoi was responding with information on MIAs, and it would no longer veto Vietnam's application for membership in the United Nations. Left open was the question of whether the issue reappeared or take more positive action.

At the United Nations yesterday, the United States and all other Security Council members, meeting as an admissions committee, recommended Vietnam's acceptance into the world organization. Some reports describe the intended action, to be taken by consensus, as a means of enabling the United States to avoid a vote in the Security Council.

An administration official, however, said the consensus procedure is not novel, and the fact is that "we joined in a recommendation that Vietnam be admitted" when the U.N. General Assembly convenes in September, Congressional committee chairmen involved had been informed in advance of the administration's intention.

Rep. Lester L. Wolff (D-N.Y.), chairman of the House International Relations Asian and Pacific Affairs Subcommittee, said of the decision on Vietnam: "It may bring them closer to the community of nations."

Wolff has scheduled a hearing today with former Secretary of State Henry A. Kissinger about the disputed postwar reconstruction aid of up to $4.75 billion promised by then, President Nixon to North Vietnam in 1973. Nixon and Kissinger say Hanoi's violation of the 1973 cease-fire in Vietnam wiped out any U.S. aid commitment.

Philip C. Habib, under secretary of state for political affairs, said on Saturday that the United States has told Hanoi it wants "to put the past behind us and make a start on normal working relations."

Speaking to the National League of Families of American Prisoners and Missing in Southeast Asia, Habib said that in the Paris talks conducted in May and June the United States " . . . proposed to the Vietnamese that we proceed at once to normalize our relations, including the exchange of ambassadors. We offered to drop our trade embargo, contingent on the opening of embassies in our two capitals. And we told the Vietnamese we would withdraw our opposition to their membership in the United Nations."

Habib said, "We believe that opening an American mission in Hanoi would further facilitate progress" in getting additional information on MIA accounting and returning the remains of prisoners, beyond agreements reached in May and in June.

"Vietnam has not accepted this proposal thus far, on the ground that it fails to provide a U.S. contribution or reconstruction aid for their country, which they claim was envisioned by past agreements."

He said U.S. negotiators have repeatedly told the Vietnamese that the U.S. view is "that we have no obligation to aid them, and that congressional restrictions specifically bar U.S. aid to Vietnam."

He said, "We continue to hope Vietnam will set aside its demand for such assistance and agree to proceed without conditions based on an interpretation of the past which we cannot, and will not, accept."