Washington — Like terriers at Uncle Sam's ankles, Vietnam veterans just won't let go of the issues that keep bothering them. Tell them that America's involvement in Southeast Asia was ''a noble cause'' (as President Reagan did in last year's campaign), and they pitch tents across from the White House, don their old fatigues, and stop eating to protest treatment by the Veterans Administration.
Set aside space in Washington for a special memorial, and many think the black receding V-shaped wall will insult their service and sacrifice. Plan a special television show for Veterans' Day that focuses on a Viet vet (as PBS is doing), and they complain to Congress that it reinforces false stereotypes.
This Sunday, Vietnam vets and their friends and families will march down Pennsylvania Avenue to the White House to draw attention to those who were prisoners or missing in Vietnam and are still unaccounted for.
It won't be a big demonstration by Washington standards. Maybe a thousand people will take part on a day when most government workers (including the President) will probably be at home watching football with the rest of America. Even the tourists will be few and far between. And it won't be a highly charged protest against Mr. Reagan.
Many Vietnam vets in fact see in Reagan a man who is more likely than his predecessor to do something about their concerns. But after years of what they see as official neglect and widespread public apathy (or worse), many others are not so sure. They are angered by the fact that the White House wanted to sharply cut federal funding for vets' counseling centers and research into the effects of the defoliant ''Agent Orange.''
''Vietnam veterans can see beyond the rhetoric today and that's why they're skeptical,'' says John Terzano of Vietnam Veterans of America, a group of some 8 ,000 vets, that pressured Congress into blocking the administration's cuts.
The memorial to Vietnam veterans would not have happened without the persistent work of one ex-soldier, Jan Scruggs. He badgered politicians and collected small contributions until he finally gained official acceptance. The group he heads has $2.5 million of the $7 million - all in private contributions - needed to build the memorial.
But like the war itself, the chosen design has generated much sharp controversy, even among Vietnam vets themselves. In testifying on the memorial's design, former infantry platoon leader Tom Carhart called it ''a black gash of shame and sorrow, hacked into the national visage that is the Mall.'' Other Vietnam vets have been critical as well.
Memorial fund leaders called a press conference this week to answer these charges. They assured critics that the design does indeed include an inscription honoring all Vietnam veterans and is not just a list of those killed.
The 200-foot-long black granite walls, said retired Marine Col. Donald Schaet , ''will reflect the sky and trees, as well as the images of the Lincoln Memorial and Washington Monument, making it truly a place for quiet and reflective contemplation of the sacrifices made by Vietnam veterans.''
If they disagree on their memorial in Washington, Vietnam veterans are united on a troubling issue that continues to linger years after the war ended.
There have been some 350 firsthand reports of Americans still held in captivity in Southeast Asia and several hundred more second-source reports. Many of these have come from Vietnamese and Laotian refugees, who report seeing American prisoners held as recently as this year.
Some 2,500 Americans who fought in Vietnam still are unaccounted for, but US officials presume all but 11 are dead. The families of the others are not so sure.
''This government's got a real problem if it ends up that people are alive over there and come back,'' says Carol Bates of the National League of Families of American Prisoners and Missing in Southeast Asia.
Defense Secretary Caspar Weinberger says the Reagan administration ''will not stop pressing'' until the US has received ''the fullest possible accounting for those who are still missing in action.'' Vietnam veterans will gather outside the White House this Sunday, just to make sure.