Washington — On the 10th anniversary of the signing of the Paris peace accords, there were poignant reminders of America's unfinished business in Vietnam. Another group of Amerasian children left Ho Chi Minh City to be reunited with their fathers in the United States. ''The long, lonely years melted away like magic,'' said one father as he greeted his Vietnamese wife and three children.
At the same time, another legacy of that troubling period was being expressed here. The children, parents, and other family members of the 2,494 Americans still unaccounted for gathered in Washington with renewed hope that their long days of waiting may be drawing to a close.
After years of less-than-full government support, they say they now see a new commitment from the Reagan administration. There is vigorous followup to continued reports from Southeast Asian refugees, who claim to have seen Americans there since the end of the war. There are new government and privately-sponsored television announcements to generate public interest and support. And the administration is asking US allies to keep the pressure on Vietnam for a full accounting.
All of this has helped along by the same spirit which impelled recent public outpourings on behalf of Vietnam veterans.
Officials are cautious in their estimate of whether any Americans unaccounted for (2,454 serviceman and 40 civilians) remain alive. ''There is no irrefutable proof,'' said Army Lt. Col. Joe Harvey, commanding officer of the Joint Casualty Resolution Center. ''But there is a significant body of evidence to that effect.''
Since Saigon (now Ho Chi Minh City) fell to the North Vietnamese in 1975, there have been 1,683 reports from Southeast Asia concerning Americans. Over 465 of these are firsthand ''live sightings'' by refugees who fled the region.
''My dad could be any one of those,'' said Martin Wright of Everett, Washington. He was 7 years old when he last saw his father, an Air Force pilot who was shot down over North Vietnam in 1967.
While US government policy holds out the possibility of survivors, some officials caution against over-optimism. One senior officer working with the issue conceded privately that he would be ''extremely surprised'' if any more than 10 Americans returned from Vietnam, Laos, or Kampuchea.
Still, the 600 people who gathered for a meeting of the National League of Families of American Prisoners and Missing in Southeast Asia are determined to press for a full accounting of their loved ones.
''We will never give up and break faith with those who never gave up for us, '' declared George Brooks, chairman of the group's board of directors and father of a young Navy pilot unaccounted for until Laotian resistance fighters turned over his remains last year.
The families also are encouraged by recent developments. Late last year, a league delegation traveled to Vietnam and Laos to talk with officials and visit aircraft crash sites. Last month, the first of what will be quarterly meetings on the issue between US and Vietnamese officials was held.
Vietnamese officials deny they are holding any prisoners or remains, and say the question is strictly a humanitarian issue, not tied to possible US aid. But they add it is difficult for the Vietnamese people to cooperate while the US remains ''hostile'' to their government.
Both the families and US officials say the Laotian government has in recent months been more helpful to those seeking information on missing Americans.
Just before visiting the Vietnam Veterans Memorial, where the names of those still missing are chiseled in black granite, the families heard from President Reagan. They applauded when he acknowledged earlier official neglect and declared that ''the intelligence assets of the United States are fully focused on this issue.''
''I'm glad to see that there's an apparent change in policy and that they're interested in resolving it,'' said M. P. Peterson of Oak Ridge, Tenn., whose son was downed by a North Vietnamese surface-to-air missile the day before the Paris peace accords were signed. ''We're just hopeful.''