Ten years ago, Nguyen, a young South Vietnamese military intelligence officer, was torn between despair and hope. On the one hand, he feared a "coming bloodbath" as the North Vietnamese Army closed around Saigon and the former South Vietnamese government crumbled. On the other hand, he recalled, as an officer whose bureau had worked closely with U.S. intelligence, "I was assured of an imminent evacuation."

But, like thousands of others, Nguyen was left behind when the last Americans departed only hours before the fall of Saigon to communist forces on April 30, 1975. Arrested soon afterward, he spent the next eight years in reeducation camps.

Today, Nguyen, who only recently managed to find a menial job, is a bitter man. Like countless other Vietnamese, he desperately wants to leave the country. Yet, as he explained in a letter passed to a correspondent here, it is with mixed feelings that he yearns to go to the United States.

"Time doesn't heal everything, most of all hard feelings," he wrote, "and for me, asking to go to the States to start my life anew is something like turning to a friend who has deserted you in hours of despair and misery. But 10 years is more than a 'decent interval,' and I'm as ever a dreamer dreaming of a white Christmas." The references were to the title of a book by former CIA agent Frank Snepp and the Bing Crosby song that signaled the final U.S. evacuation from Saigon.

He ended his letter, "Hello to the free world."

Enclosed with the letter was what American refugee officials in Bangkok call a "charm school diploma," a certificate of release from communist reeducation.

Thousands of other former members of the South Vietnamese armed forces or government have not yet received their "diplomas." And hopes are fading that they will be released anytime soon.

Those still held in reeducation centers, which former inmates describe essentially as forced-labor camps, seem to be viewed by communist authorities as the hard core of the former regime.

According to Le Duc Tho, the No. 5 man in Hanoi's Communist Party hierarchy, the "vast majority" of those sent to reeducation camps in 1975 have been released, and now "only 10,000 remain." Estimates by western diplomats in Hanoi have put the figure as high as 40,000.

"Those who remain are war criminals, not simply war collaborators," Le Duc Tho told a news conference here Wednesday. "That's why they're still there." He added, "In the course of their reeducation, they didn't get good results. When they show better results in reeducation, we will release them."

However, Tho has ruled out sending the inmates to the United States as Hanoi originally proposed more than three years ago. After Washington publicly took Vietnam up on the offer last year, Hanoi leaders backpedaled, demanding impossible guarantees that those released would not engage in opposition activities.

Since then, the Vietnamese line has hardened with the charge that those still in reeducation are guilty of "war crimes" -- although none have been tried -- and therefore cannot be sent abroad.

In fact, many Vietnamese sent to reeducation camps for long periods were members of specific units that the communists hated or feared, or had committed the crime of putting up last-ditch resistance as the Saigon government was about to fall, according to Vietnamese and western diplomatic sources.

Among those still held, the sources said, are some of the best South Vietnamese officers, charismatic men who were popular with their troops and considered good leaders.

"They're dangerous," said a Bangkok-based diplomat. "They're dangerous right now." They are still held, he said, because Vietnam's communist authorities fear they could still attract a following if they were free.

"These are people who could become competition," he said, "and Marx is a very jealous god." Singled out for retribution after the communist takeover, according to Vietnamese sources, were members of the General Political Warfare Department engaged in various kinds of psychological warfare, intelligence organizations like Nguyen's and elite units such as South Vietnamese airborne Rangers and Marines.

A colonel involved in psychological warfare who was released a year and a half ago because he was dying of cancer told U.S. refugee officials that everyone he had worked with was still being held.

Also still held are an estimated 300 to 400 South Vietnamese Army chaplains, according to one Bangkok-based refugee official.

Many who have been released from reeducation live in destitition on the fringes of society, relegated to menial jobs such as driving pedicabs.

One former captain, a company commander in the South Vietnamese airborne who said he spent eight years in reeducation after his arrest in 1975, now earns a living by giving rides on the back of his bicycle in a South Vietnamese city. He said he earns about 200 dong a month ($2 at the official exchange rate; 60 cents at the black market rate) and is the sole supporter of a wife and four children.

"Life is very hard," he said, "and we are very poor."

The daughter of another officer recounted a rare visit to see her father in a reeducation camp in North Vietnam in 1978.

"It was like a scene from 'The Bridge on the River Kwai,' " she said, referring to the film about allied prisoners of war held by the Japanese during World War II.

She said her father never had enough to eat and was constantly sick. "Since that part of the country was so poor, he couldn't even find snakes and mice to eat," she said. Eventually he was released through the intervention of a communist relative, who has since turned against the system, she said. Still in poor health from his five-year detention, he now is waiting to leave the country.

The 29-year-old son of a former Army colonel, twice sent to reeducation camps for trying to flee by boat, described an incident in which a prisoner was shot while trying to escape from a camp in Long An province.

"He had been betrayed, so the guards just waited and shot him to make him an example," the former inmate said. "They left the body there for three or four hours, then they called the prisoners to bury it in the camp graveyard," where he said there were four or five other graves.

Approximately 800 "political prisoners" were in this camp, he said, about half of them former military officers.

Today the issue of reeducation camps is apparently among the most sensitive for Vietnamese leaders. Amnesty International and other human-rights organizations have repeatedly condemned the camps and called for detainees either to be formally tried or released.

The Vietnamese concerns were evident this week when authorities took about 50 foreign journalists to a model reeducation camp near Xuan Loc in Dong Nai province.

The journalists saw a tourist complex that the prisoners had built, but not the actual reeducation camp they lived in. Brief conversations with inmates were permitted under controlled conditions.

Le Chi Khang, 45, a former Army captain assigned to the "center of military instruction," said he was sent to reeducation in 1975 for an unspecified period and arrested again in 1981 for "passing secrets to a foreign organization."

A companion, Mai Ngoc Binh, 36, a former military communications officer, said he was arrested a second time in 1983 for organizing the departures of "boat people."

Both now work in the camp's muddy zoo, cleaning cages that reek of excrement and feeding the various monkeys, birds, snakes and other animals. Both said they have no idea when they will be released and, citing camp rules, declined to discuss the criteria used to determine their state of "reeducation." Neither has been put on trial.

The camp's warden, Maj. Le Nhan of the Dong Nai province military security organization, said there were now "less than 1,000" inmates at the camp, known as K4. He stressed repeatedly that inmates were clamoring to remain in the camp after their sentences ended and presented one former South Vietnamese major who said he stayed on because "I fell in love with this camp."

Le Nhan also acknowledged, however, that there had been some escape attempts. He declined to say how many, or what the results were. CAPTION: Picture, Le Chi Khang and Mai Ngoc Binh, both former Saigon Army captains, discuss their work at K$ reeducation camp, where they look after zoo animals. ; Picture /two: Camp inmate interviewed at K4. Warden says "less than 1,000" ramain.