If you sometimes wonder why the Vietnam war refuses to fade away, consider the case of the 2,500 servicemen unaccounted for. Most are presumed to be dead, or deserters. But some 700 U.S. airmen were shot down over Laos. Could even a few of them still be alive in Laotian POW camps?

There is considerable circumstantial evidence: intelligence reports of "sightings" and satellite photographs. Ann Mills Griffiths, executive director of the National League of Families of American Prisoners and Missing in Southeast Asia, has said that, based on classified Defense Department documents, "I would bet my life" on it.

James (Bo) Gritz, a highly decorated former Green Beret colonel, already has. His recent, much- publicized forays into Laos in search of the missing landed him last week in the witness chair before a House subcommittee. A square-jawed, ramrod- straight professional soldier from central casting, Gritz recounted a four-year campaign to organize and execute a rescue operation (with and without U.S. government help). He was at once compelling and inconclusive.

But what struck me most was the questioning, and later news accounts. Somehow the tables got turned, as they usually do. Whether the issue is Agent Orange or the fate of the MIAs, the burden of proof seems always to fall on those who would question the performance of the government.

Thus committee chairman Stephen Solarz (D- N.Y.) pressed for "hard, firm, concrete evidence." Gritz replied: "I have the same evidence, sir, that might be presented by a clergyman to convince you that God exists." That wasn't good enough for Rep. Robert Torricelli (D-N.J.): "Colonel, time is either going to prove you to be one of the great heroes of our time or you are going to have a very difficult time with your conscience for raising the hopes of desperate families."

The congressman is wrong on both counts. Gritz does not need time to establish his hero credentials. That was done glowingly in a full chapter in Gen. William C. Westmoreland's memoirs. It recites some of Gritz's extraordinary Special Forces operations in the course of his four years of combat in Vietnam--including an almost impossible mission into Cambodia in 1966 to recover the "black box" of a crashed U2 reconnaissance plane.

Still less need Gritz's conscience be troubled for raising false hopes. Those hopes were raised by the same intelligence reports that first set Gritz to wondering whether the U.S. government was doing enough, one way or another, to ease the torment of "desperate families."

Gritz has been variously pictured as some sort of eccentric, compulsive, combat-soldier of fortune, and his own performance hasn't helped. He was forced to admit the other day that in fund-raising pitches for his mission he played fast and loose with the truth, not about his own heroics but about those of fellow Green Berets. Some argue that this destroys his credibility across the board.

But the central question is still whether there could be some MIAs alive. Here Gritz simply shares the view expressed by President Reagan this week: "I don't think we can afford to believe there aren't."

Gritz testified in quite specific terms about the encouragement he got, as far back as 1976, from high- ranking Army officers. His first private mission was organized in 1981 with the government's knowledge. He disbanded it after he was assured that newly elected President Reagan had "decided on an official rescue mission"--which was then abandoned.

His latest private effort, he says, was fatally compromised by a breach of confidence on the part of one of his own men. So he came up empty of "hard, firm, concrete evidence." And that's all that seemed to concern either the House committee or the ABC Evening News account: "Gritz offered little information to back his claim. . . . His credibility came apart under questioning. . . . The former Green Beret had nothing to show after three clandestine expeditions into Laos."

Maybe so. But the statements of government witnesses had little to show, as well, in answer to Gritz's theory of the case: that successive administrations have shown a marked disinclination to accept (let alone act on) anything less than the most conclusive proof that American servicemen are being held against their will in Laos.

The United States cannot be expected to reveal every measure it is taking to make good on its public commitments to give the problem of the MIAs "the highest priority." But if Gritz is right, events may shift the burden of proof back where it belongs. "A U.S. POW is going to someday walk out of the jungle and ask us why we failed to keep faith with them," Gritz believes. And the next question, he insists, will be: "What took you so long?"