Vietnam has agreed to allow Vietnamese to emigrate to join family members who already have fled the country, a United Nations refugee official said today.

An agreement between Hanoi and the U.N. High Commissioner on Refugees could lessen the exodus of "boat people," whose desperate efforts to escape have scattered them in misery around Southeast Asian seas and often led to their deaths.

Dale de Haan, deputy high commissioner for refugees, said a new program worked out with Vietnamese officials during a week-long visit to Hanoi "will regularize the departure of those who would leave anyway."

Many details remain to be worked out, but the Vietnamese decision is a major reversal of a policy that has left thousands of its citizens, almost all in what was formerly South Vietnam, with no way out except to risk the hazards of the sea and the often hostile reception upon arrival in neighboring nations.

Vietnam has pledged that former officials in the south and others who have been sent to re-education camps or otherwise detained will be eligible for emigration, de Haan said. Hanoi also dropped an earlier reservation that would have barred men ofmilitary age from leaving, he added.

Only criminals and persons in possession of state secrets will be barred from emigrating, de Haan added. The definition of criminals does not include political opponents of the government, he said.

"I think the Vietnamese are serious about this and are approaching it in good faith. They see this as a positive contribution to easing the problems of Southeast Asia," de Haan said.

The U.N. official said that he did not know how many would apply or how many would be accepted by countries with Vietnamese population but that the number might reach 500,000.

Since 1975, the United States has admitted almost 175,000 Indochinese, ations after charging that Vietnam was still mistreating its resident ethnic Chinese and forcing them to flee to China.

Analysts continued to note hints of Chinese objections to the actual location of the border, which could lead to new disputes or armed clashes. A Chinese diplomatic note on last Thursmost of them Vietnamese. Since the upsurge of Vietnamese migration, the United States has been accepting about 2,100 monthly, compared to 1,000 by France, 1,000 by Canada and 750 by Australia. These countries support the new program, said de Haan, who is a U.S. citizen.

[Deputy Assistant Secretary of State Frank Sieverty said the U.S. government "welcomes any program allowing for family reunions" and already has authorized visas that await only followup action by Vietnam. Such cases are handled independent of existing quota programs.]

This new emigration program, which will begin by summer, will not be at the expense of those refugees who are in camps or floating on ships, awaiting permission to resettle in some other country, de Haan said. The numbers of those refugees being accepted by foreign countries is not to decline.

Any Vietnamese can apply to emigrate through his local district authorities, de Haan said. These applications will be forwarded to the U.N. High Commissioner officials working in Vietnam.

Only those seeking to be reunited with their families and a smaller number of other "humanitarian" cases will be eligible. De Haan said the Vietnamese government will publicize the program in its radio broadcasts and newspapers.

The emigration program has no time limit at present, nor has it been decided what possessions or amounts of money emigrants will be allowed to take with them. Among the first to be considered will be people whose requests for immigrant visas had been approved before the April 1975 collapse of South Vietnam and those whose requests were pending.

These emigrants will have to qualify under the immigration laws of the countries receiving them but most nations give priority to relatives of citizens or permanent resident aliens. Vietnamese living abroad will be able to request that their relatives be allowed to join them.

People who apply and are refused permission to leave will not be punished or suffer hardships, de Haan said, declining to discuss what assureances he had received.

De Haan's trip to Hanoi was in response to a Jan. 12 announcement that Vietnam was willing to discusss the orderly movement of persons seeking to emigrate.

Questions about Vietnam's motives for changing its policy should be addressed to Hanoi officials, de Haan said. He added, nowever, that the officials he met were sensitive to the tragic loss of lives on the seas where boats have sunk or broken down and drifted while food and water supplies were exhausted.

Hanoi's decision acknowledges its failure to incorporate all the former South Vietnamese people into its social and economic system. The boat people have told stories of increasing inflation and food shortages and large numbers of people have been barred from getting work and government rations because of their identification with the defeated government.

De Haan said that reunion of families in Vietnam also was discussed. The U.N. High Commission has about 2,000 applications from Vietnamese who fled and now seek to return to Vietnam.