Former Secretary of State Henry A. Kissinger calls it "an absurdity" for Vietnam to claim that the United States is still bound by a 1973 proposal to give it billions of dollars in postwar reconstruction aid.

"We owe them nothing," Kissinger told a House subcommittee yesterday because "they systematically violated" the Jan. 27, 1973, ceasefire agreement between what was then North and South Vietnam "from practically the first day."

Kissinger also said he di not want "to second-guess" his successor, Cyrus R. Vance, over the Carter administration decision to support unified Communist Vietnam's admission to the United Nations. But he said "we wanted to make it more conditional" on an accounting of American servicemen missing in South Vietnam.

Now, Kissinger said, in the light of all current claims on American resources, "I would oppose" aid to Vietnam, "I would not even consider reconstruction aid until we have a full accounting" of the missing Americans, Kissinger said.

Kissinger was testifying in the two-year-old controversy over whether the Nixon administration misled Congress about aid commitments to North Vietnam in conjunction with the cease-fire accord, which brought Kissinger a Nobel Peace Prize for dipolmacy. It was his first congressional testimony on international issues since he left office.

North Vietnam, in December, 1975, disclosed what it contends was a pledge by former President Nixon in a Feb. 1, 1973, letter to supply it with $3.25 billion in unconditional reconstruction aid, plus $1 billion to $1.5 billion in normal economic aid. Nixon and other officials have said the proposal was extremely conditional, tied to compliance with the cease-fire and to congressional approval.

Rep. Lester L. Woff (D-N.Y., chairman of the House International Relationals Subcommittee on Asian and Pacific Affairs, asked Kissinger:

". . . Why did you and other members of the Nixon and Ford administrations repeatedly deny the existence, and then when that became im-letter of Feb. 1, 1973 . . . "

Kissinger, bristling at some questions by Wolff and other Democrats, said "there was nothing to cover up."

There was no offer of "unconditional aid," Kissinger said. What was mentioned in the Nixon letter to Vietnamese Premier Pham Van Dong, he said, was "a target figure," a "ballpark figure around which negotiations would take place."

Potential postwar reconstruction aid to North Vietnam was never a secret, Kissinger said, for it was first suggested by President Johnson and repeatedly mentioned by Nixon and others.

A possible $2.5 billion in aid for Vietnam was cited by Nixon in 1972, but Kissinger acknowledged that the $3.25 billion and higher figures were not disclosed. He said "maybe it should have been," but that Nixon letter was not made public until it was pried out by the subcommittee. Kissinger said it was regarded as an executive document, and the record shows that it binding aid commitment.

To support that explanation. Kissinger produced copies of what he called a 57-page document which "we handed over to the Vietnamese in Hanoi in February, 1973, "explaining in excruciating detail . . . what the American constitutional processes would be."

The compilation, as shown, is a melange that starts with the words. "The Congress plays a key role whenever funds are involved." It includes explanations of U.S. aid programs, "how a program might be structel" for Vietnam, quotations from members of Congress protesting foreign aid spending, interviews, newspaper clippings, and even a National Security Countil memorandum on Vietnam discussions with congressional leaders.

Kissinger said wryly that Hanoi's leaders "could not believe that the Congress was not a docile instrument of the administration." He said the North Vietnamese "constantly told us that Congress was a rubber stamp, and therefore this was a subterfuge."

When Chairman Wolff asked if there were any other "undisclosed agreements or promises" about aid, Kissinger said, "I cannot find such a document," and if the Vietnamese have one, "they ought to produce it."

Kissinger said, "I don't object to attempts to normalize relations" with Vietnam. But he said Vietnam's claims of an American aid obligation after massive breaches of the 1976 cease-fire and the military conquest of South Vietnam in April 1975, are "an absurdity" as well as "an insult."

He also said "I find it sort of offensive . . . that a few bodies [of American servicemen] are produced by the Vietnamese in order to appeal to AMerican public opinion."

Republicans on the subcommittee strongly supported Kissinger throughout the questioning in which he was rarely hard-pressed and dominated the scene, with touches of his old-style quips and repartee.

Early in the hearing, Rep. J. Herbert Burke (R-Fla, said, "It seems to me now that this question has been fully resolved." He added, challengingly, "I hope that this mater doesn't become a political type of thing - which it seems to me it is becoming." Chairman Wolff effusively commended Kissinger for his "voluntary testimony" as the hearing ended after two hours, with no sign that Kissinger had been bruised by the encounter.