Vice President Bush announced yesterday that the Laotian government, in a further sign of cooperation with the United States, has agreed to excavate a second crash site to search for the remains of U.S. servicemen shot down over Laos during the Vietnam war.

Bush made the announcement during a speech to the 16th annual convention of the League of Families of American Prisoners and Missing in Southeast Asia. His remarks provoked loud applause from 800 people attending the convention in Alexandria.

The announcement comes just five months after the remains of 13 servicemen shot down more than a dozen years ago over Laos were recovered in the first joint U.S.-Laotian operation.

State Department officials said yesterday that the second crash site mentioned by Bush yesterday is one of five suggested to the Laotians by the United States. However, officials said they do not know yet which site will be chosen. The sites involve crashes that occurred between 1967 and 1972.

Bush said yesterday the excavation would occur during the dry season in Laos, which runs approximately from December to April.

He attributed the new development to efforts underway since 1982 to improve relations between Laos and the United States and to the Reagan administration's attempts to track down the remains of the nearly 2,500 American servicemen still missing in Indochina, including 556 in Laos and 1,375 in Vietnam.

The Vietnamese government announced this month that it plans to turn over the remains of 26 more missing servicemen, the largest group to be returned since the war ended a decade ago.

Bush said, "At last America has a president -- and, may I say, a vice president -- who has pledged that, of this war, we will write no last chapter, we will close no books, we will put away no final memories until all your questions about the missing and about possible prisoners of war are answered."

Echoing earlier comments by President Reagan, the vice president said that recovery of MIAs is "the highest national priority" and added, in a remark that drew sustained applause, "If we can get hard evidence that Americans are still held in Vietnamese prisons, we pledge to do whatever's necessary to get them out."

Earlier yesterday, Rep. Stephen J. Solarz (D-N.Y.) got a less friendly response when he focused on the question of whether Americans may still be prisoners of war. Solarz, chairman of the House subcommittee on Asian and Pacific affairs, expressed doubts about assertions made by Robert Garwood, a former POW convicted of collaboration with the enemy, that Americans are still being held prisoner in Vietnam.

To scattered boos, and shouts of "Rambo! Rambo!" Solarz said, "We've attempted to track down every lead, and respond to every allegation, and we will continue to do so. But we must be careful not to raise false hopes." The "Rambo" cry referred to the current film in which a character played by Sylvester Stallone singlehandedly rescues American POWs who the U.S. government tells him do not exist.

George Brooks, League chairman, apologized to Solarz for the interruptions, saying, "We can disagree, but we should disagree agreeably."