Bruce D'Agostino, liaison officer for Veterans of the Vietnam War Inc., wants to go on a goodwill mission to Vietnam. He hopes at least to get information on Americans missing in action and still unaccounted for -- and at best to bring some back home with him. D'Agostino says he has been getting positive signals from Hanoi officials recently, and he thinks there won't be a better time to pursue the quest that has meant anguish and frustration for so many in the 12 years since the war ended.

D'Agostino sent telegrams to the Vietnamese foreign ministry, requesting a meeting to discuss ''humanitarian issues.'' On Oct. 23 he met in New York with Nguyen Dang Quang, first secretary of the Vietnamese mission to the United Nations.

D'Agostino said Quang agreed that the time for negotiation has come. He said that the Vietnamese diplomat told him: ''We have tried many times, many ways, and gotten nowhere with the U.S. government.''

A few days after that meeting, Quang told us that his government has no American prisoners of war. He also said he didn't think there are any in Cambodia, most of which is occupied by Vietnamese troops. But Quang said he knows that Cambodia does have some remains of American soldiers, and that the Phnom Penh government is willing and ready to turn them over to the United States if asked.

Current unofficial estimates place the number of surviving American prisoners in Southeast Asia at anywhere from 400 to 700, and the U.S. government has been strangely lethargic in its efforts to locate them and obtain their freedom.

What does the Reagan administration have to say about D'Agostino and his proposed mission? D'Agostino doesn't know, for the simple reason that no administration official will meet with him.

D'Agostino called Charlotte DeMoss of the White House Public Liaison Office several times during the week of Oct. 12 to arrange a meeting, but got nowhere. So on Oct. 20 he called Elizabeth Proctor in the Private Sector Initiatives Office. She said she might be able to arrange a meeting the next day.

D'Agostino flew from Boston to Washington that night and left a message for Proctor to call him. She phoned him the next morning to report that she had set up a 3 p.m. appointment with Patricia Barnett, assistant director of the Private Sector Initiatives Office.

Fifteen minutes later, however, Proctor called back and canceled the afternoon appointment with her boss, and suggested that D'Agostino contact Col. Howard Hill at the Pentagon instead. Hill's secretary refused to give D'Agostino an appointment, and suggested that if he wanted information on POWs he should call the Pentagon's public affairs office.

Exasperated by this runaround, D'Agostino tried to call Proctor again. When he finally reached her -- on the third try -- she told him he would have to ''go through channels'' and submit any proposal on a visit to Vietnam in writing to the Pentagon. Alexander Almasov, a State Department official who deals with Vietnamese issues, said it seemed ''strange'' to him that D'Agostino got the cold shoulder at the White House. But he added that U.S. officials expect to meet with the Vietnamese shortly to discuss the POW situation and said the administration usually tries to discourage private initiatives because they often raise false hopes among the families of POW/MIAs.

In 1985, we obtained a tape in which then-national security adviser Robert McFarlane admitted that the administration hadn't made thorough investigations of reported POW sightings. He said it was his opinion that some American soldiers are still in Indochina.

''There is quite a lot of evidence given by people who have no ulterior motives and no reason to lie, and they're telling things they have seen,'' McFarlane said on the tape.

D'Agostino's frustration with official Washington led him to arrange his meeting with Quang. He said he found the Vietnamese diplomat's attitude encouraging, and that he hopes to visit Vietnam soon.

After his meeting with Quang, D'Agostino tried to telephone White House officials again -- but again he was brushed off. He said he was hoping he could get the administration's support for his mission, but acknowledged that this seems unlikely.

''If we do, great,'' he said. ''If not, we'll do it anyway.''