The families and friends of hundreds of U.S. servicemen missing in Southeast Asia since the Vietnam war are gathering here today for their 16th annual convention with more hope of progress but more internal division than ever.

Even as the government of Vietnam is promising new efforts to account for 1,375 men listed as missing and unaccounted for there, the largest families' group, the League of Families of American Prisoners and Missing in Southeast Asia, is beset by criticism from what some congressional observers are calling "the Rambo faction."

It includes members and returned veterans who recently have become much more vocal in asserting that league leaders trust the U.S. government too much, and that Vietnam may still be holding U.S. prisoners.

League members have just elected several new board members who say they will push a search for those prisoners harder and question the U.S. government more than the outgoing board did. Their assertiveness, if it comes to dominate the board, could affect the U.S. negotiating stance in Southeast Asia.

The dissident faction's name derives from the current hit move, "Rambo: First Blood, Part II," which tells of a Vietnam veteran's effort to find and free POWs who the U.S. government tells him do not exist. Although the families say their concern predated the movie by many years, their suspicion of the motives of the U.S. and Vietnamese governments is reflected in the film.

Vice President Bush will keynote the convention's general session Friday, and national security affairs adviser Robert C. McFarlane is scheduled to speak Saturday, which is POW-MIA Recognition Day. Both officials are expected to praise Vietnam's recent promises to return 26 bodies and account for six more as a triumph for the Reagan administration's negotiating efforts.

Few of the families question that the communist government in Vietnam, beleaguered by major economic worries and bogged down by determined resistance to its occupation of Cambodia, may be serious in its avowed "willingness to resolve" the cases over the next two years. The "Rambo faction," however, questions just what that may mean.

"Does that mean really resolving it or just clearing the books? It's a big difference," said Ann M. Hart, 42, of Pensacola, Fla. She obtained a temporary restraining order in California July 5 against releasing what the Pentagon said were the remains of 13 servicemen, including her husband, Thomas, on grounds that there was no real basis for the identification.

Like many others, she thinks Vietnam has warehoused as many as 400 bodies and is releasing them individually at politically helpful moments. This is such a moment, she said.

Vietnam desperately needs the renewed world trade and investment that would be sure to follow normalization of relations with the United States, but the Reagan administration has insisted that any talk of renewed ties must follow a resolution of the MIA question and a withdrawal of Vietnamese forces from Cambodia. The new initiatives are widely viewed as a possible first step along that long road.

"There is a lot more reason for encouragement now," league executive director Ann Mills Griffiths said in an interview. "This is the first time we've ever had a time schedule." She dated renewed interest in the MIA issue from the 1981 election of President Reagan, who had done radio broadcasts on the issue of Korean War MIAs in the 1950s and began a public awareness effort when he became president.

"Before that it was raised when convenient . . . as 'a hopeful byproduct of negotiations,' " Griffiths said. "I've seen the tears in the president's eyes when I've talked to him about it. I know his commitment is real, and the Vietnamese know it, too."

She said that the question of live POWs has always been and remains the league's and the administration's top priority, and that her critics "may have some personal resentment" against her because of her close association with high officials and her access to classified data. The critics, she said, "have no credibility on this issue."

Former Carter administration officials have said their efforts to negotiate were just as sincere as Reagan's but were hampered by Vietnam's trial of an alleged U.S. spy and then by Vietnam's invasion of Cambodia in 1978 and the wave of "boat people" who fled Vietnam in 1979.

Rep. William M. Hendon (R-N.C.), a member of the House Task Force on American Prisoners and Missing in Southeast Asia, has obtained 51 cosponsors for proposed legislation to set up a commission headed by H. Ross Perot that would try to determine whether there are American prisoners in Southeast Asia and recommend measures to free them. Perot, a wealthy computer software businessman, freed some of his employes who were held briefly in Iran and has visited Vietnam several times in pursuit of MIA clues there.

The league opposes the measure as unnecessary duplication of effort.

New board member John W. Parcels, 40, a retired major who was held prisoner in Laos and North Vietnam from 1970 to 1973 and who is a member of the "Rambo faction," said in an interview that the bill would be a good start.

"We're going to get progressively more radical," he said. "We've been very patient."