Retired Army general John W. Vessey, back from a high-level mission to Hanoi, said yesterday that an agreement by Vietnam to resolve cases of missing American servicemen will be judged by "the action it produces."

Vessey briefed President Reagan on his talks last week in Hanoi and told reporters only firm answers from Vietnam can help resolve doubts about Americans still missing in Southeast Asia.

"As we made it very clear to the Vietnamese, there are a number of questions that have to be answered before the American people -- and particularly the families -- will be satisfied that there are no live POWs or that they're not holding out on the issue of remains," Vessey said.

At the end of three days of talks last week, U.S. and Vietnamese officials agreed to speed the search for MIAs and said the United States, in a switch of policy, had agreed to address "urgent humanitarian concerns" of Vietnam.

A similar delegation, headed by Richard K. Childress of the National Security Council staff, opened talks yesterday in Laos on revived MIA cooperation. Some 1,776 Americans are listed as missing in Vietnam and another 550 in Laos.

Laos permitted joint excavations of U.S. warplane crash sites in 1985 and 1986, but hopes for further excavations this year were dashed as both Vientiane and Hanoi put all cooperation on hold, apparently in response to internal criticism they were receiving nothing in return for their assistance.