HO CHI MINH CITY, JULY 4 -- Vietnam said today it has agreed to a meeting in Hanoi with retired general John W. Vessey Jr., President Reagan's special envoy, in an effort to end the impasse in talks over the fate of American servicemen still missing from the Vietnam war.

A knowledgeable Vietnamese government official, speaking here on condition that his name not be used, said the Vietnamese have given the Americans a letter accepting Vessey's proposed visit with "no preconditions" and agreeing that the talks center only around "humanitarian issues" and not the thornier issue of normalizing relations.

The Vietnamese had insisted earlier that any high-level talks between Washington and Hanoi be "open-ended," including possible discussion of normalizing relations, if the Vietnamese cooperated in accounting for the 1,800 missing U.S. servicemen. The Reagan administration has insisted that any talks be limited to a "humanitarian agenda," including the fate of Americans missing in action, or MIAs.

An American negotiating team returned from Hanoi in late May without winning a Vietnamese commitment to accept a Vessey visit. Reagan named Vessey, a former chairman of the Joint Chiefs of Staff, to try to break the impasse that developed when Hanoi suspended the talks last fall.

The Vietnamese had expressed frustration that while they cooperated with the Americans in trying to resolve the MIA question, the Reagan administration took what Hanoi considered a series of hostile steps in return, including signing a military agreement to stockpile arms in Thailand.

Vietnamese Foreign Minister Nguyen Co Thach, in an interview Friday, said Hanoi would accept an agenda for future talks with Washington that concentrated solely on humanitarian issues. But he suggested that Vietnam intended to raise a wide range of issues under that category, including property damage and injuries suffered during the period of American military involvement.

"They can first raise the issue of MIAs. We can raise the question of, for example, the victims of war," Thach said. "If we can help the Americans on MIAs then the Americans can help us heal our wounds of war."

Thach said Hanoi's insistence on discussing humanitarian issues besides MIAs did not amount to a precondition. "If the United States says that its wounds of war are more important than the Vietnamese wounds of war, then it's one-way traffic," he said.

In the 40-minute interview, conducted here in the former capital of the American-backed South Vietnamese government, Thach said he saw no urgency in normalizing relations with the United States, despite Vietnam's cash-strapped economy and the possibility that normalization might bring badly needed American aid, trade and investment.

"We exist. We have existed for 2,000 years without normalization with the United States," Thach said. "We can exist some years more without normalization. If Vietnam wants normalization and the United States has no interest, then it is of no use. You must have two partners, just as in friendship and in love."

Thach also made it clear that he thinks Vietnam's international isolation is ending even without the United States.

Vietnam has embarked on a broad program of economic reforms, he said, with an emphasis on encouraging exports and a proposed investment code that would allow foreign firms to enter into joint ventures here and control up to 100 percent of some enterprises.

Thach said the country conducts about $400 million in trade with Japan and with the noncommunist countries of Southeast Asia, despite a nominal agreement to isolate Vietnam economically following its 1978 invasion of Cambodia. Vietnam ousted the brutal Khmer Rouge regime of Pol Pot and installed a new government in Phnom Penh sympathetic to Hanoi.

Some economic analysts and diplomats in Hanoi put the trade figure even higher, with Singapore as the primary conduit for Southeast Asia. Several Japanese firms are said to be discussing joint ventures once the investment code is finalized at the end of this year.

Thach called the western strategy of isolating Vietnam "a double-edged sword," increasing Hanoi's dependence on Moscow for aid. His remarks clearly suggested that Vietnam, in the midst of near-revolutionary economic reforms, would not mind diversifying from its principal aid supplier.

Thach also repeated Hanoi's often stated pledge to withdraw all of its troops from Cambodia by 1990 despite continuing attacks from Khmer Rouge guerrillas, questions about the effectiveness and reliability of the newly created Cambodian Army, and serious doubts about the ability of the Vietnamese-installed government to survive without Hanoi's military might.

Thach was slightly more emphatic than in the past, insisting that only foreign military intervention -- presumably from China -- could convince Vietnam to change its plans for a total withdrawal. The 1990 date, Thach said, was based on a long-term Vietnamese strategy "not to withdraw at once, but not get bogged down" in a guerrilla war that some western analysts have referred to as "Vietnam's Vietnam." Thach said, "1990 is enough time to give responsibility to the Kampuchean {Cambodian} people. It is enough time to give Kampuchea an army, an administration."

"We have foreseen many eventualities after our withdrawal," Thach said. "The first eventuality is that the Kampuchean Army can maintain the 1987 situation with the Vietnamese. The second eventuality is that the situation would be worse, but it could not be worse than the Philippine situation."

The Philippines is currently battling 20,000 communist insurgents. Thach said the Khmer Rouge forces are not as effective as the Philippine rebels and are incapable of staging large-scale attacks.