SO BURDENED and embittered is the relationship between the United States and communist-ruled Vietnam that it becomes extremely difficult for even the smallest matter lying between them to be treated in calm, dispassionate terms. There is such a small matter, however, and it needs to be treated in that way. It concerns the personal misfortune of William Mather, a 41-year- old sailor who currently happens to be the only American known to be detained in Vietnam.

Bill Mather, Yale '66, served four years in Vietnam as a Navy officer and stayed on in Asia, working in marine salvage and the like. Last July, preparing to wrap up his affairs in Asia, he set out to sail his 80-foot schooner, the So Fong, from Singapore to Hong Kong, where he hoped to sell it. His plan was to stay in international waters, avoiding going too far to sea in the typhoon season (with a small crew of three adults and two children) and at the same time steering clear of pirate operations in the southern Philippines (no arms were aboard). The Vietnamese who seized him said he was in their territorial waters.

His family speculates that the government of Vietnam, in its mood of deep suspicion, thought the CIA had sent him to map its waters in advance of an invasion. But there is not the slightest evidence that this is the case, and he has not been so accused. The crew members (French and Australian) were eventually freed. Bill Mather is still detained -- in acceptable circumstances, the family believes -- nothwithstanding assurances of his early release that have been passed to some of the many Americans and foreigners who have inquired quietly after him.

One American wanderer: After everything, with everything, it is hard to think of what might induce the Vietnamese to let him go. Bill Mather is a small fish caught in a large net. If he is released, there will be no great turn in relations between the United States and Vietnam. But if he is released, the many Americans who will hear about it will give it notice, will nod, will say to themselves, yes, the Vietnamese finally released that American, the fellow in the yacht, someone who was doing them no harm. The Vietnamese should do it.