The presidential commission that leaves today for Hanoi to ask Vietnam about the 2,550 U.S. servicemen still unaccounted for in Southeast Asia also will visit Laos, the White House announced yesterday.

Approval from the Laotion government came hours before the five commission members met for 45 minutes with President Carter, Secretary of State Cyrus R. Vance and others from the State and Defense departments.

The White House said efforts to extend the trip into Cambodia have been unsuccessful so far. But United Auto Workers President Leonard Woodcock, head of the commission, said later that no request to visit Cambodia had yet been made.

There is a possibility that the Cambodian ambassador in Hanoi might be contacted, Woodcock said.

Carter told the group he hopes this mission " . . . will lead to complete normalization of relations . . . We have come through the Vietnam wat years with a lot of scars, psychological and others, which need to be healed. There remains now, I think, no hatred in the American poeple."

Jerrold Schecter, spokesman for the National Security Council, said the group expects to spend about three days in Hanoi after arriving Wednesday. He said there were no details on whether it would go directly to Vientianne fro Hanoi or whether a separate trip would be made.

In a statement issued after the meeting, Carter said he hopes "that this step we are taking will meet with a positive response . . .," but "we recognize that information may never be available on many" of the missing.

Woodcock echoed that thought, saying in a statement: ". . . We should not expect the impossible."

The commission will be the first official U.S. group to visit Hanoi since a select committee of Congress did so in December, 1975. Five American journalists will accompany the commission.

It will raise the question of missing servicemen in the hope that the Vietnamese will be forthcoming, a U.S. official said. No other specific issues will be brought up by the commission, but the members will report to President Carter on any discussions initiated by the Vietnamese.

An encouraging report by the commission would make it more likely that Carter might ease the U.S. trade embargo or take other major steps toward normalizing relations with Vietnam.

"Nobody can predict what is going to happen in the future, at the response of the Vietnamese government," Carter said last Saturday. "I think they want to re-establish relationships with our country."

The United States has quietly taken four small steps to indicate its willingness to end the near-total estrangement between the two nations.

According to U.S. officials, Washington raised no objection to a U.N. Development Program's proposed five-year, $44 million aid program for Vietnam.

Similarly, the United States did not attempt to block procedural administrative measures in the International Atomic Energy Agency that open the way for Vietnam's membership. Under the Atoms for Peace Program, South Vietnam acquired a small reactor from the United States.

Americans removed the reactor's core just before Communist troops captured it in the rapid collapse of the Saigon government in 1975.

The United States also raised no objections to a recent World Bank mission to Vietnam that will lead to a loan program from the International bank, nor did it object to a recent $40,000 shipment of rice organized by U.S. humanitarian organizations.

In addition to Hanoi's desire to talk to the Carter administration, U.S. officials point to a recent Foreign Ministry statement as an encouraging sign.

During the almost two years since the North won the war and brought about the reunification of Vietnam, Hanoi has insisted that the United States must contribute to postwar Vietnamese reconstruction.

Vietnam made public a Feb. 1, 1973, letter from then-President Nixon to Premier Pham Van Dong promising $3.25 billion in postwar aid.

However, the United States has taken the position that Hanoi massively violated the 1973 Paris cease-fire agreement and that all promises of aid are thereby void.

Congress, reacting to the Communist military offensive that brought victory, legislated a ban on aid to Vietnam that prevents the administration from changing its position even should it want to.

On March 3, a Foreign Ministry spokesman said the United States "should" and "ought to" provide aid. U.S. officials note that in the past Hanoi said "must."

In addition, the Hanoi spokesman did not refer to the Paris agreement, which U.S. officials took as helpful since they regard the agreement as no longer in effect.

If Hanoi insists that it will not give a more complete accounting of missing Americans unless it receives aid, U.S. officials said, that will leave U.S.-Vietnamese relations frozen.

The Carter administration does not insist on a complete accounting because one may be impossible.

However, the administration knows that Hanoi has the remains of at least 12 Americans and has additional information that it could provide.

The presidential commission, therefore, will be following the path marked by the select committee, chaired by Rep. G. V. (Sonny) Montgomery (D-Miss.), which said a total accounting is not possible.

Montgomery is a member of the commission that left yesterday. The other members are former Senate Majority Leader Mike Mansfield, former U.N. Ambassador Charles Yost and Marian Edelman, director of the Childrens Defense Fund. State Department officials and five reporters will accompany them.

Montgomery said last week that his committee "started a dialogue which should be continued." He hopes for "a decent accounting of the MIA," he said.

Of the 2,550 unaccounted for, all but 750 have been declared dead by their families or the Pentagon as a result of the circumstances of their being lost as witnessed by others.

Carter, Secretary of State Cyrus R. Vance and Defense Secretary Harold Brown met with the commission yesterday before it departed for its three days in Hanoi.

Carter spoke often during the presidential campaign of moving toward normalization of relations with Vietnam.

The commission's trip fulfills as implicit Carter campaign pledge. During the second presidential debate he attacked President Ford for not sending a mission to Hanoi to seek further information on missing Americans.

He called the omission "one of the most embarrassing failures of the Ford administration."