Hanoi is willing to allow U.S. government and military personnel to accompany Vietnamese teams investigating reports that living Americans have remained in Vietnam since the Vietnam War, U.S. congressmen said in Bangkok yesterday.

Vietnamese Deputy Foreign Minister Hoang Bich Son told the congressmen that the U.S. personnel could include officials from the administration and from the Joint Casualty Resolution Center in Hawaii, which has handled remains of American servicemen turned over by Hanoi, according to a delegation staff member in Bangkok interviewed by telephone.

State Department officials in Washington, who had only seen press reports, said the reports, if confirmed, would be a significant development and another indication of Hanoi's willingness to expand cooperation with the United States to press ahead to resolve the MIA issue. Since August, the Vietnamese have been making concessions on the MIA issue in hopes of establishing diplomatic relations with the United States and ending Hanoi's international political and economic isolation following its December 1978 invasion of neighboring Cambodia.

Washington has been careful to avoid any movement toward diplomatic ties in its dealings with Vietnam on the MIA issue. But U.S. officials have indicated that by cooperating, Vietnam can be in position for normal relations with the United States once the Cambodian conflict is settled.

The delegation, headed by Rep. Gerald B. Solomon (R-N.Y.), was told by Son that three Vietnamese teams are investigating reports of sightings of live Americans. Son made clear, according to the delegation staff member, that he was not referring to Vietnam's earlier investigations into at least three reports of sightings of Americans. Those probes found that the persons involved were not Americans.

Son, who met twice with the nine-member House delegation, said "there's a ray of hope" about living Americans in Vietnam, the staff member said.

Rep. Benjamin A. Gilman (R-N.Y.) said at the Bangkok press conference that Son issued a "standing invitation" to the U.S. government and military to join the investigation, according to news agency reports.

In addition, Son appeared to indicate more flexibility on the possibility that living Americans were in the country. He said that there was a possibility some Americans who were either "postwar subversives" or who had "illegally infiltrated" the country after the war might be in areas outside Hanoi's control, such as in the mountain regions, the congressmen told reporters in Hanoi before traveling to Bangkok.

Last August, the Vietnamese tacitly acknowledged the possibility that some former U.S. servicemen still may be living in Vietnam, after having denied it for years. Last month, during a visit to Hanoi by Assistant Secretary of State Paul D. Wolfowitz and Assistant Secretary of Defense Richard L. Armitage, Washington and Hanoi agreed to intensify efforts to resolve the MIA issue within two years and investigate reports of "live sightings" of Americans.

The congressmen also announced that Hanoi has confirmed that a technical meeting set for later this month would take place on Feb. 26 and 27. Son said Hanoi would produce information about 70 Americans whose bodies have not been recovered, the congressmen said. Hanoi had said originally that it would provide information about 50 such Americans. After investigating these additional 20 cases, the Vietnamese found at least 14 sets of remains of U.S. servicemen.