Rebecca A. Nguyen's letter {May 28} on Vietnamese citizens seeking to be united with family members in the United States raises valid questions about actions and inactions in both countries that have led to the long delays in processing cases of family reunification.

However, the backlog of 20,000 applications that Mrs. Nguyen cites in the Orderly Departure Program exists because processing of these applications broke down after the government of Vietnam began to issue exit visas to large numbers of persons in none of the categories for whom the program was designed -- Amerasian children, divided families and political prisoners.

I continue to urge the Reagan administration to give priority to family reunification cases in processing claims under the Orderly Departure Program. As Mrs. Nguyen states, our government needs to show greater flexibility in dealing with applications that are often difficult to document. We also need to assign higher diplomatic priority to our relations with the Vietnamese government on this issue. The administration's recent appointment of former chairman of the Joint Chiefs of Staff, Gen. John W. Vessey, as a special representative to Vietnam is a step in the right direction.

On the question of political prisoners, Vietnamese officials offered five years ago to release 8,000 to 9,000 Vietnamese now held in "re-education camps," if the United States would be willing to accept them as immigrants. The administration accepted the offer in 1984, and it is the government of Vietnam which has failed to respond further. Many of these cases also involve family reunification, and it is a humanitarian tragedy of unacceptable dimensions that families such as Mrs. Nguyen's continue to be divided so many years after the Vietnam War ended.

EDWARD M. KENNEDY U.S. Senator (D-Mass.) Washington