Secretary of State George P. Shultz yesterday warned the families of missing Vietnam war veterans as well as Hanoi that the United States was not prepared to "trade information on our missing men for economic aid" and would only discuss "humanitarian issues" at forthcoming talks there.

Shultz said the administration was "pleased" that the Vietnamese government has agreed to receive a new presidential envoy, retired Army general John W. Vessey Jr., the former chairman of the Joint Chiefs of Staff, and that it also had agreed that the two sides should not link humanitarian issues with outstanding political problems between Hanoi and Washington.

"We intend to honor that agreement and expect Vietnam's leaders to do the same," he told the 18th annual meeting here of the National League of Families of American Prisoners and Missing in Southeast Asia.

But Shultz warned the families that Vessey might not find it possible "to move the issue forward" of recovering the remains, or discovering the whereabouts, of the more than 2,400 U.S. servicemen still listed as missing.

"Recent press reports indicate that Vietnam is raising the concept of humanitarian cooperation as a 'two-way street,' including economic assistance," he said. "Humanitarian reciprocity is one thing, but any attempt to trade information on our missing men for economic aid is another.

"We cannot agree to this," he added.

No date has been set yet for Vessey's trip to Hanoi.

Shultz appeared to be referring to the comments made in a recent interview with The Washington Post by Vietnam's foreign minister, Nyugen Co Thach, who said that if the United States wanted help in solving the problem of missing servicemen "then the Americans can help us on our wounds."

Thach said his government was under pressure to get something in return for its cooperation with Washington and suggested the United States had certain "obligations" to Vietnam because of heavy war destruction.

Vietnam has previously sought to link talks on the missing Americans to a general normalization of U.S.-Vietnamese relations and American reconstruction aid; the administration has refused any such linkage.

In Singapore last month, Shultz reiterated the U.S. position that there would be no move toward normalization of relations with Vietnam "until a settlement has been reached . . . which involves the withdrawal of Vietnamese forces from Cambodia."

Yesterday, Shultz said the administration looked forward to "reengagement on a political level" with Vietnam but that the withdrawal of its forces from Cambodia "must precede it." Vietnam invaded Cambodia and imposed a pro-Vietnamese regime there in 1978.

Shultz said there were other "humanitarian" issues the administration wanted to discuss with Hanoi, including the fate of Amerasian children, family reunification and prisoners in "reeducation" camps. Vietnam has said it has "similar concerns" to raise with Vessey, adding, "we are prepared to address all those matters which are clearly humanitarian in nature."

However, Hanoi so far has refused to discuss the issue of Vietnamese political prisoners or to permit the emigration of Amerasians, who primarily are the children of U.S. servicemen.

Meanwhile, State Department officials said Hanoi has agreed to allow the United States to resume interviewing Vietnamese who have applied to emigrate to the United States after suspending immigration interviews in January 1986.