Vietnam has sternly warned a visiting American delegation that there will be no further cooperation in resolving the issue of missing American servicemen in Indochina as long as the US uses the MIA issue as a ''weapon against Vietnam.''
But Vietnamese Foreign Minister Nguyen Co Thach showed a hint of optimism by declaring that President Reagan is easier to deal with than former President Carter.
The US group, led by Richard Armitage, a deputy assistant secretary of defense, held two days of meetings this week with Vietnamese counterparts in Hanoi. All it could win was an agreement that specialists from both sides will continue to meet ''when necessary'' and that a delegation of Vietnamese specialists will, at an undetermined date, repeat earlier visits to the joint casualty resolution center in Honolulu. The center oversees American efforts to recover the remains of MIAs.
The Vietnamese were particularly critical of a public accusation from Washington last July that Hanoi was ''holding back'' on the question of America's missing in action.
One Vietnamese official said Hanoi was at that time beginning to move toward a resolution of the MIA issue that could have included joint Vietnamese-American teams in Vietnam. The official claimed that Vietnam had backed away from the concept of joint teams after Washington's July accusations.
But despite the rift, Vietnamese officials insist their efforts to find the remains of American servicemen will continue.
Some 2,500 American soldiers are listed as missing in Vietnam. All but 10 have been classified as dead, but their remains are unrecovered. There is no estimate of the number of Vietnamese missing during 30 years of war in Indochina.