WASHINGTON (AP) _ After 18 months of inaction that sparked a new and dangerous surge of refugees fleeing in rickety boats, Vietnam has agreed to resume the Orderly Departure Program for citizens seeking to emigrate to America, the State Department said Friday.

The Vietnamese decision lifts a January 1986 suspension of permission for U.S. officials to go to Vietnam and interview applicants who want to fly to the United States by way of other Southeast Asian countries.

Interviews are now expected to resume ''in the not-too-distant future,'' said Deborah Cavin, a State Department spokeswoman.

The Hanoi government's willingness to resume the program was disclosed by Hoang Quoc Thin, deputy director of consular affairs in the Vietnamese foreign ministry, during talks on Monday and Tuesday with Bruce Beardsley, a U.S. diplomat based in Thailand.

''We're terrifically happy about this,'' said a U.S. official, speaking on condition of anonymity. ''This means people will come out in a safe way and not in those horrible boats.''

On the down side, the official said, was Vietnamese refusal to permit processing of political prisoners and Amerasians - mostly the children of U.S. servicemen in the Vietnam War.

The State Department will continue to press the Vietnamese on the issue, the official said.

The United States has been beseeching Vietnam for months to resume the Orderly Departure Program, which was set up in 1979 and has resulted in the safe emigration of 58,000 Vietnamse citizens with relatives in the United States.

Vietnam suspended interviews for prospective emigrants 18 months ago on grounds there was a backlog of people who had been processed by the Vietnamese authorities but had not yet been accepted for entry into the United States.

The original purpose of the program, set up by the United Nations High Commissioner for Refugees in 1979, was to stop the need for people to sneak out of Vietnam in frail boats, many of which sank or were attacked by pirates in the waters between Vietnam and nearby countries.

The program seemed to do the job because the number of boat people declined, but since the January 1986 freeze on orderly departure interviews, figures on boat escapes have creeped upward.

In May, the United Nations reported the arrival of 1,326 boat people in Thailand alone, the highest monthly total since June 1981.

When the program resumes, teams of U.S. officials drawn from the State Department and the Immigration and Naturalization Service will interview applicants.

The United States has a list of 94,000 people with letters of introduction from relatives already in the United States and Vietnam has a list of 19,000 to whom exit permits have been granted but who are awaiting processing for departure. The initial group of those interviewed by the American teams will be from the Vietnamese list.

State Department officials declined to speculate on any motives Hanoi may have had for allowing the interviews to resume, beyond Vietnam's stated reason that the backlog has been cleared.

The United States has been searching for signs of Vietnamese willingness to reduce several sources of friction that have remained between the two countries since the end of the Vietnam War 12 years ago.

Besides orderly emigration, the United States wants Vietnam to end its occupation of Cambodia and to be more helpful in accounting for the estimated 2,400 American servicemen still missing from the Indochina conflict.

Last month, when a new set of leaders assumed positions at the top of the Hanoi government, Secretary of State George P. Shultz and foreign ministers of nations in the region voiced hopes for a new Vietnamese willingness to deal with Western concerns.

Since then, the Vietnamese have agreed to let retired Gen. John W. Vessey, a former chairman of the Joint Chiefs of Staff, come to Hanoi as President Reagan's emissary to discuss the fate of the missing American servicemen.