Many staunchly anti-Communist Vietnamese refugees in the United States have lent a hand to the Communists in rebuilding their country recovering from three decades of war. The Communists have cheerfully collected foreign currencies the low-income refugees have sent to relatives in Vietnam, while always calling refugees "traitors."

In this erratic, uneasy cooperation between the two never-under-the-same-roof foes, each side pursues its own goal. The refugees want to help their cherished relatives alleviate misery under the Communist regime. The Communists want to accumulate foreign currencies that they desperately need for reconstruction.

The amount of money flowing from the refugees to their relatives is estimated at millions of dollars this year, and the Communist have conducted an elaborate and systematic drive to encourage the flow.

Vietnamese refugees are surprised at the relatively unsharp scissors Vietnamese censors have used against outgoing mail. Various letters implying harsh conditions under the new Communist rule in South Vietnam have reached the outside world. Some refugees believe that the Communists do not want to impose rigid control in the hope that the compassion-stirring letters might attract some money home.

According to refugees who recently escaped. Communist cadres often urge people to seek financial support from relatives overseas. "Write your sister and tell her to send money home." Vuong Si Thanh quoted a Communist cell leader as saying when handing him a letter from a Virginia resident. Thanh, 27, a native of a coastal city 200 miles northeast of Saigon, arrived in the United States in March after having spent 19 1/2 months under Communist rule.

"To encourage and urge relatives abroad to transfer money home is the duty of everyone," the Saigon newspaper Tin Sang said in article that was broadcast by the Communist radio in August. "We must not only advise overseas compatriots," the paper continued, "and create conditions for them to maintain sentimental links with their families and homeland, but must all help the government collect more foreign currencies to develop production and industrialize the country."

According to Tin Sang, persons receiving foreign currency are given a bonus that, depending on the currency, ranges from 25 to 70 per cent over the official exchange rate.

A few privileges are also granted to some persons who distinguish themselves by bringing in much foreign currency. A widow was presented a medal of "national reconstruction" and given a secial status as a middle-level cadre, enabling her to buy food and commodities at government stores, according to Le Kim Ngan, a former college professor who left Vietnam 11 months ago. The woman, Ngan said, receives monthly allowances from some of her children in Europe.

Partly at the encouragement of Communist authorities and partly because of severe economic conditions, letters from Vietnam oftens solicit money, medicines and clothes. The refugees' hearts soften when reading their relatives' discrete portrayal of their rough life in the newly reunified nation, and they are eager to be of some help. "I'm lucky to have escaped from the Communist yoke and have started the new life here but I won't be happy, if I couldn't help my beloved relatives living in Saigon," said a former air force major, now a Maryland county employee.

Sending money, a practice banned by U.S. laws, is done openly by the Vietnamese community. Understandably, the refugees who do it do not want to be identified.

Until relations between the United States and Vietnam are established, money and clothes sent from here must find a zigzag path via the refugees' friends in countries with relations with Vietnam. France, with its large Vietnamese community and its long-time connections stemming from nearly a century of domination of the Southeast Asian nation, is the most common channel.

Canada, Australia, Japan and Hong Kong are other conduits. "I sent my wife and three small children $50 tentatively last year via Australia," said a Florida pizza parlor employee, formerly an army captain of the now-defunct American-supported government. "I'm going to send them $300 soon through an acquaintance in Hong Kong."

Almost every refugee family with relatives still living in Vietnam has sent money, according to a businesswoman with contacts in the 6,000-member Vietnamese community in Virginia.

As the refugees have become more established, the amount of money being sent is increasingly larger. A California woman, whose husband was a colonel, said she had sent $100 last year and $400 some months ago to her sister and three brothers in Hanoi, whom she has not seen for 23 years. She works in a beach hotel as a maid.

The sums involved may seem small in the eyes of Americans, but to Vietnamese receivers they are really big. The pizza shop employee's wife, a typist, exchanged her $50 into 120 Vietnamese piastres, or "as much as nearly three times her salary," he said. With the new bonus for foreign currency, the exchange ratio becomes higher. A 42-year-old refugee said that his brother, a middle-level government employee in Hanoi where the bonus has been in effect for years, converted $80 into 399 North Vietnamese piastres, "equivalent to his eight months' salary." A North Vietnamese piastre is reportedly worth 80 per cent of a South Vietnamese one. More than a year following the official reunification, Vietnam still remains different monetary units in the two parts of the country.

It is difficult to know the amount of dollars streaming from the refugees in their relatives in Vietnam through various channels. In 18 telephone calls to Vietnamese names chosen at random in Washington area phone books, seven persons said they have sent money, and two were reluctant to answer the question. The calls indicate that about one-third of the refugee households have supported their relatives in Vietnam.

Of 145,000 refugees in the United States, 30,682 are heads of households, according to a U.S. government report. If one-third of the families send their relatives $100 a year - a minimum sum the Vietnamese say they usually send - the annual amount would be about $1 million. From the disclosures by the interviewees, however, the figure may reach several million dollars this year.

Rumors circulate among the refugees that when their relatives are summoned to receive money, authorities ask questions about the senders. Other rumors said that the money receivers are "advised" by officials to deposit part of the money saving accounts from which they can withdraw it only for important reasons.

Nguyen Cong Hoan, a defector from the Communist National Assembly last March, said in an interview that the Communists are temporarily permissive to the money receivers at this stage when they badly need foreign currency. But in the long run, he said, the regime will "never" tolerate a privileged minority" who live on the support from persons whom the government calls traitors-in-exile.

Despite the rumors, many refugees continue sending money. "As long as my relatives receive my money, I feel I still have an obligation to send them a little sum at times. Affections and compassions must overcome politics in this case, " a thoughtful refugee said pitifully.