Slow progress seems to be being made in the United States' decade-long effort to induce Vietnam to render an accounting for the 2,441 American servicemen still considered missing in action since the Vietnam war. The highest- level team that the American government has sent to Hanoi since the fall of Saigon reported a Vietnamese undertaking to resolve the MIA issue in two years and to check reports that live American prisoners or deserters remain in the country.

Do the Vietnamese understand the importance of forthright delivery on these promises? It is not simply that Americans feel morally obliged to pursue all information on the men who did not come back from the war and to retrieve the remains of those who died there. Often Hanoi has appeared to be toying with the issue: playing hard to get and concealing known facts in order to pump up American impatience and exact a price in diplomatic recognition.

The MIAs aside, a great mutual mistrust has marked the attitudes of the two countries. It has been fueled by Hanoi's violation of the Paris peace accords, its mistreatment of those in the South who became its citizens by conquest, and its aggression against Cambodia. Thus did Vietnam undermine the effort at reconciliation that many Americans, including President Carter, were ready for in the 1970s. In the 1980s, Hanoi suggested it would allow Amerasian children to depart and permit the resettlement of "reducation" camp inmates, but its performance on these and other humanitarian issues has dragged. Americans have good reason, on the MIAs, to see if Hanoi is as good as its word.

It is commonly said that Americans understood Vietnam poorly. Vietnamese communists have not understood the United States well either. After their victory in 1975, they made an offensive demand for American "reparations." They failed to anticipate how the developing American interest in Peking was bound to work against them. Supposedly noninterventionist, the regime invaded Cambodia; supposedly nationalistic, it gave strategic bases to the Soviet Union. Having isolated itself from the broad international community, Vietnam found aid, trade and development lagging. A decade that could have been devoted to rebuilding left the country still in miserable shape.

A contemplation of this unhappy result is what has apparently brought Vietnam's rulers to grasp its interest in the MIAs. It is a humanitarian question, but the resolving of it would surely signal Hanoi's readiness to ease political differences as well. Americans would be bound to pay attention -- and to be bitterly disappointed if Hanoi did not honor its commitment.