One year after the unification of the two Vietnams, life in the southern part of the country and the former capital, Saigon, seems not so much oppressive as harsh.

The Hanoi government has put pressure on the southerners to make them coming to Communist ways. But this pressure has been applied gradually and according to a well-defined plan.

Gathering details of how this plan is taking effect is somewhat easier than hearing about xenophobic Cambodia and to a certain extent. Laos. An extensive foreign diplomatic corps lives in Hanoi and there are some insulates in Saigon, now called Ho Chi Minh City.

But the main source of information about life in the south are letter's from residents to relatives living aboard, the government's own news media and the continuing flow of refugees.

These bits and pieces of information making their way to Bangkok, a major Vietnam-watching outpost, but unlike Cambodia, that there apparently have been few executions of anti-Communists.

Instead, the regime has arrested, isolated or, as Prime Minister Pham Van Dong put it recently, "rehabilitated" active supporters of the U.S. backed Nguyen Van Thieu government. This is a euphemism for psychological or physical punishment.

Groups singled out for particularly harsh treatment are former senior military officers, activist clergy and private businessmen.

Between 40,000 and 400,000 southerners, many of them senior officers in the former southern army, have been sent to "rehabilitation" or "reeducation" camps since the Communist victory more than two years ago.

According to reports from some of the 15,000 refugees who have fled Vietnam since the end of the war, life in these camps is extremely hard. An unknown number of internees have died from malnutrition and disease, they say.

A refugee who reached Thailand last month said in an interview that his two brothers, who had been officers in non-combat positions, were arrested just after the fail of Saigon. "No member of our family has heard from them since. We don't know where they are or if they are dead or alive," said the refugee, who asked that he not be identified for fear of jeopardizing his relatives still in Vietnam.

Both Buddhist and Roman Catholic clergymen are having difficulty. As they've done in the North, the Communists are working toward eliminating organized religion in the South, or at least reducing its effectiveness.

A Western diplomat who served recently in Hanoi said that the authorities have moved to bring the clergy under their influence by granting them privileges and by creating such groups as the Patriotic Buddhist Organization.

Although Communist authorities in Hanoi often make a point of conducting Western visitors to church services, this diplomat said, young Vietnamese were being taught to shun religion and most congregations comprised only older people.

According to several Western and nonaligned diplomats here who observe Vietnamese affairs, the authorities have been rounding up clergymen who had been active opponents of the Thieu government simply because it was feared that they might organize opposition to the Communists.

A Vietnamese monk who has lived in Bangkok for six years said two Catholic bishops, Tran Huu Thanh and Nguyen Van Thuan, were arrested and died in jail of "unknown causes." Other Catholic clerics still held include Archbishop Nguyen Van Binn; the bishop of Danang, Pham Ngoc Chi; and the bishop of Vinhlong, Nguyen Van Diep.

The monk showed personal letters and circulars from Saigon, Paris and several cities in United States that contained these and the names of others reportedly arrested in the last few months.

Among them were Thich Tri Quang, a leader of Saigon's An Quang pagoda, who had been labeled "the red monk" by Thieu and was perhaps the most important religious figure to oppose the former government. Others were the leader of the Hoa Hao sect. Luong Trong Tuong; most leaders of the Cao Dai sect, and a highly venerated wandering monk, Nguyen Thanh Nam.

There have been some news reports in recent months of large-scale self-immolations of Buddhists monks.

Some observers, however, believe that these reports are somewhat exaggerated. One diplomat whose government maintains an embassy in Hanoi said he had seen only one confirmed report of six nuns and three monks burning themselves to death last year in Cantho, a city in the Mekong Delta.

Another, and perhaps the most obvious, target for the new government is the capitalist system in the South. The Ho Chi Minh City Communist Party organization recently announced that it intended to "complete the transformation" of private industry and commerce and turn all farms into collectives in the next two years.

Private business still goes on in the South, but the government's statements, monitored in Bangkok on radio and over the Vietnam News Agency wire, make it clear that entrepreneurs are being squeezed out of the economic picture.

"Capitalists" and "speculators" are frequently blamed for the massive economic problems that have plagued southern Vietnam since the flow of U.S. aid was halted.

Although the economy is still described as "multi-sectoral," the Communists emphasize the takeover of the private sector by the state. The Vietnam News Agency has warned businessmen that they must realize that socialism is the "inevitable law of the Vietnamese revolution."

The news agency and Vietnam Radio mention from time to time that the government faces considerable problems, particularly in the South. Usually hough, reference to difficulties is couched in reports of progress, such as increases in irrigation canal mileage or electrical power wattage.

Some observers believe that the drive toward socialism in the agricultural sector is partly to blame for a 45,000-ton rice shortfall this year, although they concede that Vietnam was struck by a severe drought and unusual cold in the rice-producing areas of the Mekong Delta and the Red River.

One of the problems the Communists inherited from the Thieu government was unemployment. According to official statements, some 1 million Saigonese were out of work when the Communists came to power. Since then, half of these people have been sent out of the city "new economic zones," collective farms reclaimed from the jungle.

Another 200,000 "volunteers" have also gone out to the zones. Some refugees reaching Thailand have told of extreme hardships in these areas, including shortages of food, tools, housing material and medicines.

One Eastern European diplomat described the new economic zones as a "brilliant storke of organization talent" by the Communists. But, he conceded, "There probably have been some deaths from malnutrition and disease. This is only natural because of bureaucratic shortcomings in a project of such scope."

Some observers in Bangkok and in Western capitals believe that the impressive organizational ability of the Vietnamese government is responsible for keeping the flow of refugees out of the country relatively low.

Compared with an estimated 15,000 Vietnamese, mainly the so-called "boat cases" who have reached various Southeast Asian countries, 25,000 Cambodians and 88,000 Laotians have fled to Thailand.

Refugee workers here believe, though, that the basic reason for the relatively small number of Vietnamese leaving is that their journey is much more hazardous.

"I'm sure that if it was as easy to cross the South China Sea as it is to swim the Mekong from Laos to Thailand, we'd have many, many more Vietnamese making a run for it," one international agency officer said.