The United States and Vietnam exchanged detailed work plans this week on resolving within two years the issue of American servicemen missing or unaccounted for since the Vietnam War, U.S. officials said today. They said the talks in Hanoi were "the most substantive and productive" yet held in several years of negotiations on the issue.

"There has been an obvious high-level government decision in Vietnam to remove this as an obstacle" to relations between Washington and Hanoi, said Ann Mills Griffiths, a member of the U.S. delegation and executive director of the Washington-based National League of Families of American Prisoners and Missing in Southeast Asia. "The Vietnamese have recognized that it's in their national interest to resolve this issue," she told a news conference here.

Vietnamese officials who conferred with the U.S. delegation during their visit to Hanoi Wednesday and Thursday also tacitly acknowledged the possibility that some former American servicemen may still be living in Vietnam, according to U.S. sources who did not wish to be identified. They said the Vietnamese disclosed that they have investigated at least three reports of "live sightings" of Americans recently, but found that the persons involved were not Americans.

In the past, Vietnamese officials have consistently denied that any American prisoners of war are still being held or that any U.S. deserters are living in Vietnam voluntarily.

The Reagan administration takes the position that although the presence of living U.S. prisoners of war in Vietnam cannot be proved, the possibility that some may still be held cannot be ruled out.

The administration also believes, based on the testimony of a Vietnamese mortician who fled his homeland in 1979 and other unspecified corroborating evidence, that Hanoi is withholding the remains of hundreds of U.S. military personnel killed in the Vietnam War, U.S. officials said.

Vietnamese officials have denied stockpiling any remains of U.S. servicemen.

Since the war ended in 1975, Vietnam has returned 99 sets of remains subsequently identified as being those of Americans. An additional 26 remains returned on Aug. 14 are currently being analyzed by U.S. specialists in Hawaii and may turn out to be the biggest single repatriation of American remains from Vietnam to date.

In today's press conference, Griffiths, whose brother is among those unaccounted for in Vietnam, said progress was made at the Hanoi meeting despite the last-minute shelving of plans for "high-level talks" on the missing-in-action (MIA) issue.

Vietnamese Foreign Minister Nguyen Co Thach proposed such talks last month through an intermediary, and Washington made plans to send to Hanoi a delegation led by Assistant Defense Secretary Richard Armitage and including Assistant Secretary of State Paul Wolfowitz, National Security Council staffer Richard Childress, Griffiths and three other U.S. officials.

However, Thach, who had been expected to lead the Vietnamese side, then went on a visit to Moscow, and both Armitage and Wolfowitz withdrew from the trip amid fears that Hanoi was reneging on its own proposal. The Hanoi visit went ahead at a lower level with Childress as head of the U.S. team.

In the end, "the level of the delegation was basically irrelevant," Griffiths said, since Thach's deputies in the Foreign Ministry evidently had the authority to move forward on the issue.

She said one of the "most striking features" of the talks was the "absolute absence of any linkage whatsoever to larger political issues" such as establishing diplomatic relations between Hanoi and Washington, ending a U.S. trade embargo, unblocking international aid for Vietnam and settling the conflict in Cambodia.

In past MIA talks, the Vietnamese have routinely tried to raise such issues, but the U.S. side has declined to discuss them on grounds that accounting for the missing Americans is a separate, humanitarian matter.

Nevertheless, the U.S. policy line is that by cooperating on the MIA issue, the Vietnamese can "preposition themselves" for normal relations with the United States once the Cambodian conflict is settled.

Vietnam invaded neighboring Cambodia in December 1978 and overthrew the brutal communist Khmer Rouge regime the following month. Since then, about 160,000 to 180,000 Vietnamese occupation troops have been fighting Khmer Rouge guerrillas and two noncommunist Cambodian resistance groups. A large majority of United Nations members, including the United States, China and Southeast Asia's noncommunist states, have annually demanded the withdrawal of Vietnamese troops from Cambodia.

Some western diplomats view Vietnam's latest moves on the MIA issue as an adjunct to the maneuvering by Hanoi and its foes for diplomatic advantage going into this year's U.N. General Assembly session.

In any case, Griffiths said, "we now look forward to a greatly accelerated pace of cooperation between the two governments" toward Hanoi's recently announced goal of resolving the MIA issue within two years. She said she was particularly struck by the Vietnamese side's unexpected presentation of a specific work plan on the subject in response to a U.S. plan submitted a week before the visit.

U.S. officials said there were some "common elements" in the work plans and that the two sides would now try to agree on a final program. The Vietnamese plan called essentially for "unilateral action" on their part, while the U.S. side assumed more joint activity would be needed, the officials said.

Although senior Vietnamese officials, including Politburo members Le Duc Tho and Thach, in April specifically ruled out joint U.S.-Vietnamese excavations of crash sites of downed American aircraft, the U.S. sources said the Vietnamese left that possibility open in the talks this week. In the past, the Vietnamese have said normalization of relations was a prerequisite for such joint excavations.

In February, a team of U.S. and Laotian specialists jointly excavated a crash site near Pakse in southern Laos and recovered the remains of 13 missing U.S. airmen. Laos and the United States have maintained diplomatic relations despite the communist takeover in Vientiane in 1975.

Griffiths said that during a one-day stop in Laos Tuesday before flying to Hanoi, the U.S. delegation received confirmation of a Laotian commitment to allow another joint excavation during the upcoming November-to-May dry season.