WASHINGTON (AP) _ Vietnam has agreed, after an 18-month freeze, to process citizens seeking to emigrate to the United States, the State Department said today.

Vietnam agreed to resume interviews for the Orderly Departure Program, which allows emigrants to fly to the United States by way of other Southeast Asian countries, said department spokeswoman Deborah Cavin.

She said the Vietnamese decision was revealed to U.S. officials at a meeting in Hanoi on Monday and Tuesday and that processing under the program would begin ''in the not-too-distant future.''

The Orderly Departure Program was arranged in 1979 through the United Nations High Commissioner for Refugees to stop the flood of refugees fleeing in boats, many of which sank or were attacked by pirates in the waters between Vietnam and neighboring nations.

Vietnam suspended the U.S. portion of the program in January 1986 on grounds there was a backlog of people who were processed by the Vietnameses side, but not accepted for entry to the United States.

The United States had denied such a backlog existed and had pressed the Vietnamese to resume processing.

The decision to do so was relayed by Hoang Quoc Thin, deputy director of consular affairs in the Vietnamese foreign ministry, to a delegation headed by Bruce Beardsley, a diplomat in the U.S. Embassy in Thailand.

Most of those in the Orderly Departure Program are relatives of Vietnamese who have already come to the United States.

At the same time, the Vietnamese rejected a U.S. request that political prisoners and Amerasians - mostly the children of American servicemen who served during the Indochinese War - be included in those permitted to depart.

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. 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.

''The purpose of the Orderly Departure Program is to provide a safe, viable alternative to dangerous clandestine departures from Vietnam,'' Mrs. Cavin said.

Since 1979, she said, 125,000 Vietnamese have left under the program, 58,000 for the United States.

The program had effectively stemmed the tide of boat people, but in recent months, while the program was suspended, there had been a new surge of boat departures. The United Nations reported the arrival of 1,326 boat people in Thailand in the month of May, the highest monthly total since June 1981.