With a few seconds of silence, the Security Council today approved Vietnam's application for membership in the United Nations, which had been blocked for two years American vetoes.

Much as a minister calls for anyone objecting to a marriage to speak up and then pauses as briefly as appers decent before going on the ceremony, Security Council President chen chu of China asked if there were any objections to approving Vietnam's application by consensus.

The silence that followed lifted Vietnam over the Security Council hurdle on its fourth try, as U.S. Delegate Donald McHenry voiced no objection.

McHenry said the United States was prepared to vote in favor of Vietnam had the issue been put to a vote, but the consensus procedure is no unusual when the Security Council considers requests for membership.

Vietnam will become a member this fall when the General Assembly meets and votes on its application. Last year, 124 General Assembly members voted for Vietnam before the Ford administration cast the U.S. veto in the Security Council.

Vietnam's observer at the United Nations, Dinh Ba Thi, addressed the Security Council after its action today and called on the United States to fulfill its obligations under the 1973 Paris peace agreements to contribute billions of dollars for postwar reconstruction of Vietnam.

World public opinion supports this Vietnames demand, Thi said, noting that the delegates of many socialist nations had called on the United States to aid Vietnam during the day-and-a-half discussion of the membership application.

"It is a matter of honor, of conscience, of responsibility which the United States cannot shirk under any pretext," he told the Security Council.

Former U.S. Secretary of State Henry Kissinger told a congressional committee yesterday that promises of aid the Nixon administration made in 1973 were not binding because Vietnam had violated the terms of the Paris agreement.

McHenry told reporters after the Security Council session that all questions of U.S. reconstruction help would have to be discussed in Paris, where Washington and Hanoi havehad two rounds of talks on bilateral issues.

Congress has forbidden aid to Vietnam and the Carter administration has explained to Vietnames that no aid payments can be made until Congress is persuaded to change is stand.

The United States was the only one of the 41 nations that address the Security Council over two days on Vietnam's application that did not elaborately prase the Hanoi government.McHenry' statement yesterday was also the shortestr during the often repetitive series of speeches. It lasted about 90 seconds and noted that Vietnam would incur the responsibilities as well as the privileges that accompany being a member of the United Nations.

"We look forward to working with Vietnam, as with all other members in this body, to bring about a new era of peace, cooperation and friend, not only in oue bilateral relations but in our work together in the United Nations," McHenry said. He represented the united States for Ambassador Andrew young, who is in Europe.

Angola, Cuba, East Germany, Bulgaria and other pro-Soviet nations were most pointed in their denunciations of the United States during the discussion.

Angolan Delegate Elisio de Figuciredo, whose government is supported by Cuban troops, said the United States had had no right to be within thousands of miles of Vietnam at the time of conflict.

Kam Thong Boulom, the delegate from Laos, said his nation will support Vietnam's efforts to get reconstruction aid from the United States and added that his government stands ready to improve its relations with the United States once it also receives U.S. money to rebuild.