Vietnam has been claiming successes lately against persistent armed opposition by small insurgent groups. It appears to be growing frustrated, however, with what diplomats say is continuing "passive resistance" in the south.

According to official news reports from Hanoi, about 60 Montagnards--members of a hill tribe in central Vietnam--have defected in recent months from the opposition United Front for the Liberation of Oppressed Races following a major government crackdown. The front, known by its French acronym FULRO, has been fighting for the independence of tribes in the Central Highlands for 20 years.

More troublesome for the government, however, according to diplomats who have visited Vietnam recently, are the activities of southern Vietnamese who--since the 1975 takeover by the North Vietnamese--continue their passive resistance against Communist rule of South Vietnam and Hanoi's policy of "socialist transformation" of the region.

This concern was reflected in an unusual article published in November in Tap Chi Quan Doi Nhan Dan, the Vietnamese Army's theoretical journal.

Strongly attacking "enemies" in southern Vietnam who "oppose socialism," the monthly said such activities as the large black market trade in Ho Chi Minh City (formerly Saigon) and the influx of western goods mailed by refugees to 180,000 families in Vietnam were intended to "sap our economy."

In an extraordinary listing, the journal said the government's "enemies" included 468,000 members of the former South Vietnamese Army; more than 72,000 administrative personnel and police of the old Saigon government; 3,000 leaders and 450,000 members of disbanded political parties; about 400,000 Roman Catholics, who it said make up 13.6 percent of Ho Chi Minh City's population; 60,000 Buddhist monks and 480,000 Vietnamese Chinese accounting for 14.7 percent of the city's inhabitants.

The article said that in 1978, 81 percent of these Hoa, or overseas Chinese, asked to leave Vietnam and that most of them still in effect reject Vietnamese citizenship by refusing to accept "people's identity cards."

In unusually sharp language, the journal said that "bourgeois Hoa," encouraged by China and the United States, "are still the biggest destroyers of the economy of Ho Chi Minh City." Among other opponents, it said, were "reactionaries acting under the cover of religion."

According to the latest U.S. State Department report to Congress on human rights in Vietnam, thousands of Catholics in central Vietnam have been forcibly relocated or required to work on government construction. It said the teaching role of the Catholic Church has been restricted and many churches and all but one seminary in the south have been closed.

The report also said the religious activities of 30,000 Vietnamese Moslems of the Cham minority have been curbed. Moslem schools and associations, it added, have been closed, links with foreign Moslem organizations broken and pilgrimages to Mecca forbidden.

The Vietnamese journal also said that international enemies were trying to undermine the Army by discouraging people from performing military service, exaggerating the hardships of the Army, inciting soldiers to desertion and organizing the theft or destruction of equipment.

Western diplomats said these charges reflect the unpopularity in the south of Vietnam's war in neighboring Cambodia and problems with draft dodgers in Ho Chi Minh City.

But the diplomats were puzzled by the article's most startling assertion: that "reactionaries" backed by China and the United States had plotted to "establish secret bases to launch a long guerrilla war in remote areas and foment rebellions." In a place identified only as "District N," the journal said, "We discovered, pursued and captured an enemy organization that was plotting an armed rebellion." Among those caught, it said, were persons from "the ranks of state personnel working in an important place."

According to western diplomats who recently visited Vietnam, including Ho Chi Minh City, there have been no signs of any military movements or of any plot against the government. However, they said, rumors were circulating about the arrest of several senior figures.

The diplomats see the article as a reflection of unhappiness in the Army with departure from Marxist economic orthodoxy in the south and as a justification for a possible purge.

Recent visitors report that the private economy in Ho Chi Minh City has defied efforts to control it and that the government there has allowed private enterprise to profit from joint ventures with the state.

Among the joint ventures are relatively swanky shops, cafes and nightclubs that pack in customers despite high prices by Vietnamese standards, diplomats said. They said some officials in Hanoi apparently fear that cadres from the impoverished north will become tainted by assignments in the south and lose their revolutionary fervor.

Another concern, the diplomats said, is resistance by peasants to collectivized farming in the Mekong Delta, a resistance considered more disturbing than the armed insurgency in the Central Highlands.

"People are just not producing--they're not working in factories and they're not growing rice," said a western diplomat who visited the south recently. "People are resisting the government by not working hard. That kind of resistance is more important than FULRO will ever be."

Information about the rebel group in the Central Highlands is sketchy, but Indochina watchers here believe it numbers only a few hundred fighters with little organization and constitutes no serious threat to Vietnamese security. Even less is known about an offshoot called Dega, believed to be made up of members of the large Rhade hill tribe north of Dalat.

According to diplomats here, there have been signs of cooperation between the Montagnards of the United Front for the Liberation of Oppressed Races and Dega and the Communist Khmer Rouge guerrillas fighting Vietnamese forces in Cambodia. The Khmer Rouge reportedly are fairly strong in the remote northeastern region of Cambodia bordering the Central Highlands and are believed to have supplied some equipment and possibly training.

According to the official Radio Hanoi, seven of the front's leaders surrendered. Subsequently, Hanoi said, at least 52 Montagnard rebels defected.