FOR THE two years before and the 10 years since the final American withdrawal from Vietnam, the communist victors in Hanoi have toyed with one of the few Vietnamese matters still of deep interest to a broad American public. We refer to the 1,000 or more American servicemen believed still to be missing or otherwise unaccounted for in Vietnam and to the lesser number lost elsewhere in Vietnam-controlled Indochina.

Since 1973, with elaborate and grotesque calculation, the Vietnamese have doled out the remains of 99 American servicemen. Their evident purpose has been to bargain for aid, diplomatic relations and generally a full return to international society. They seem to have thought that by thus playing cynically on the sentiments of the American people, they could win concessions otherwise beyond their grasp.

In fact they have failed. In 1985 Vietnam remains to a considerable extent an international pariah for its treatment of the American MIAs, among other human rights issues, and for its sponsorship of a harsh occupying regime in neighboring Kampuchea, or Cambodia.

Therein lies the possible importance of an agreement Vietnam has just reached with the United States to take the necessary steps to resolve the MIA issue within two years. The agreement was reached without any of Hanoi's usual linkage of the MIAs to economic and political questions, it is reported, and it was accompanied by release of what Vietnam says are the remains of 14 American servicemen, the largest number repatriated at one time.

Has Hanoi made a fresh judgment of the worth of improving relations with the United States? Its delivery on the MIA issue will provide a telling part of the answer, as will its response to the diplomatic initiative on Cambodia being taken, with American approval, by the friendly nations of Southeast Asia.

From, among other things, the testimony of a Vietnamese mortician who fled in 1979, American officials believe that Hanoi has stockpiled the remains of hundreds of U.S. military personnel killed in the war. Such reports underline the burden on Vietnam to meet considerably higher standards of disclosure than closed communist societies customarily allow. If Hanoi hopes to get the full benefits of cooperation on this issue, however, it will have to address the range of American suspicions that its past conduct has stirred.