It's been 20 years since Don Luce first went to Vietnam.Many people who attend his lectures weren't born then. Ngo Dinh Diem and Nguyen Van Thieu are unfamiliar names to them.

The antiwar movement ended for some when helicopters lifted the U.S. ambasador, his flag and the last of the Americans out of Saigon. Luce (who, Amb. Graham Martin said in his rambling explanation of the war to Congress, should get "the main credit-or blame-for ending the war") planned to go home to Vermont to farm.

Instead, the movement never ended for Luce. He lives much as he did during the war, with three roommates in a crowded apartment, spending about $100 a week beyond the expenses that he gets from those who ask him to speak.

Time magazine once wrote that Luce was to the Saigon government what Ralph Nader was to General Motors. Now, he gives 1,200 or more speeches a year (often five a day), traveling across the country, talking about lessons of Vietnam-and the dangers of new Vietnams in Iran, the Philippines and Thailand.

Luce, who was expelled from South Vietnam after he led a congressional delegation to the tiger cages in South Vietnam's largest prison and had resigned as director of the International Voluntary Services in Vietnam to protest U.S. Vietnam policy, concentrates on Asian countries for the New York based Clergy and Laity Concerned, an organization that dropped the words "About Vietnam" from its name after the war ended and continued in operation.

He expanded his beat to include Iran because he came to believe that the United States was heading into another Vietnam there.

Iranian students here besieged him with their fears and frustrating about U.S. policy in Iran and Luce says: "The least I could do was go to investigate."

President Carter complained last month that his intelligence agencies failed to appreciate and warn him of the extent of opposition to Shah Mohammad Reza Pahlavi.

Luce has been describing the vehemence of anti-shah feeling since he returned from his first Iran trip last January. "Vietnam is good background," Luce says.

He was amazed how openly Iranians denounced the shah to him despite their fear of police reprisals. The scope of anti-shah activity which built up later in the year culminating in massive marches last weekend didn't surprise him.

He took his Iranian hosts presents that met his requirements that they be cheap and impossible to obtain in Iran-photographs of the shan crying as he stood on the White House lawn withCarter and tear gas fired at anti-shah demonstrators blew in his face.

For Luce, the U.S. government's claim that the shah is in trouble because he has brought too many reforms too fast is nonsense. "The slums of Tehran are full of farmers who were driven off the land by land reform," he says of one shah reform. "When it came, they didn't know-and weren't told-how to fill out the forms."

Luce says the outbreak of anti-shah demonstrations has not made him more optimistic about the chances of avoiding armed struggle in Iran. The United States is still committed to keeping the shah in power, Luce says, and the demonstrators will become more frustrated if their demonstrations have no effect.

If that happens, Luce says, "Demonstrations will recede and we will be told that the shah has weathered a storm, but it will only mean the people are taking time to get out the machine guns."

Luce raises a comparison between Iran today and Vietnam in February 1965 when the Vietcong mortared Pleiku Airbase, killing eight Americans, and the U.S. retaliated by beginning the bombing of North Vietnam. There were 23.600 Americans in Vietnam then. There are 45,000 in Iran now.

"What would we do if Americans were killed?" Luce asks. What did we do when the Mayaguez was reported captured by Cambodia? Because we should be concerned about American casualties and because we should be concerned about removing American provocations, Luce says, we ought to withdraw from Iran.

He watched U.S. made tanks with clasped hands of friendship on their side crush Buddhist monks in Vietnam. He marvels that the tear gas canisters in Iran are labeled "Made in U.S.A."

You'd think we'd at least know enough to label them made in the U.S.S.R.," Luce jokes.

"I think that as a country we have to deal with the fact that American weapons in Iran are being used against unarmed people who have decided that they must take to the street," Luce says.

Not only Iran, but the Philippines and Thailand pose dangers for the United States, Luce believes. "In Indonesia, South Korea and Taiwan, the anger is just as deep," he adds, "but the people still hope that out policy [of support for the government] might change."

Born in East Calais, Vt., 44 years ago, Luce has been to a lot of places since he first reached Vietnam as an agriculturist with a degree from Cornell.

In Henry Kissinger's White House office in 1973, he heard the then White House national security adviser say: "I assure you that in six months the Vietnam war will be over. Give me six months." Luce did, and then he paid another visit.

Druing a 1973 visit to North Vietnam, he was taken to the former U.S. Marine base Camp Carroll, just south of the border dividing the two Vietnams and then occupied by Communist troops. "They were raising chickens in the bunkers," Luce remembers.

In Taiwan this year, he asked a factory owner why only young teenage girls were working there. "We used to have boys, but we found girls are more docile and obedient and besides they're cheaper," is the reply Luce got.

In 1977, he went back to Saigon for the first time since his expulsion in 1971. Some of the survivros of the tiger cages gave a party for him. He also went to My Lai for the groundbreaking of a hospital and met two of the six people who survived the massacre. "What do you say when you meet the survivors of the My Lai massacre?" Luce ways.

Vietnam, Luce argues, is doing a better job respecting human rights now than the U.S. supported Saigon government did. Although thousands continue to flee Vietnam, Luce believes they are driven mostly by economic conditions, and he would like the United States to aid Vietnam.

"No country has the right to shoot its citizens," Luce replies when asked about Cambodia. He has sought a Cambodian visa to see the country for himself and has been turned down. "There's no way your can get me to defend that government," he say.

Every once in a while Luce is mentioned in newspapers, but fund-raising and getting media attention have become harder since the Vietnam war ended. Now, he is less likely to be mentioned because of what he says than because he has been caught up in a demonstration.

In Manila, he was knocked to the ground with a group of Filipino nuns by a water cannon. In New York, he was dragged out oa banquet for the shah's wife after he interrupted her speech to criticize the shah.

For that banquet earlier this year, Luce wore a tuxedo for the first time in his life. He rented it on 42nd Street and was dismayed to find the trousers had no belt loops and that he had no suspenders. He spent a lot of time tugging up his pants, Luce remembers, laughing.

He refused to stand up for the Iranian national anthem and one of his table companions thought that meant Luce might understand his daughter who had just run away from home. "I was trying to do some education with my tablemates," Luce remembers, smiling. "I told him to go to some peace demonstrations with his daughter."

When other people stood up one by one to shout protests against the shah, Luce thought he should join them.

"But I didn't want to stand up. It didn't seem dignified. Besides there were a lot of men with guns. I was a little scared. Angry protest is not my style, I said to myself," Luce wrote of the banquet.

Finally, he stood.

"While we sit here in luxury, people are being tortured in Iran," Luce shouted. "While the Empress Farah speaks of the great writers of the past, the writers of today are in jail. I have just returned from Iran where I met a woman who had cigarettes extinguished on her body during interrogations."

By the time he'd said that, Secret Service agents were chasing him around a table near the center of the room. Luce decided to shout a farewell slogan, but he had been working on Thai problems.

"Stop torture in Thailand," he shouted.

Before he could correct himself, the agents caught him. With his arms pinioned, he no longer could tug at his tuxedo pants and they fell lower and lower as he exited from the ball room.

Not all Luce's contacts with the Secret Service have had comic aspects. After Luce returned from Iran, he found that the Secret Service had interviewed his mother and his roommates.

An agent asked his mother whether he was prone to violence. His roommates were asked whether he carried weapons.

Luce sits in the cluttered offices Clergy and Laity Concerned and considers the question of why he doesn't stop, after almost 20 years, and walk away from his movement.

Why, after all that time, does he want to be a human rights reminder in a country where not very many people are paying attention?

Luce remembers feeling guilty in his first months in Vietnam because people came to him with their problems about what the United States was doing and he brushed them off, saying: "I'm an agriculturist. I don't want to get political."

He has been political ever since. In Vietnam, he was helping farmers plant sweet potatoes and then Americans were defoliating the sweet potatoes. "I went from growing potatoes to writing about tiger cages because my sweet potatoes got defoliated and I got angry. It's almost that simple," he says.

Luce doesn't believe in shouting. As he said about standing up at the Empress' banquet, he feels there's something undignified about it, and Luce has dignity.

But, underneath his level conversation and quiet humor is the sweet-potato anger. Companies that convince uneducated mothers in underdevelopled nations that expensive infant formulas are better for their babies than mother's milk makes him angry. So do American firms that expose their foreign workers to hazards they aren't allowed to get away with in the United States.

His involvment in Asia is as an America trying to argue what Americans, not Asians, are doing. Luce would prefer a world in which a Filipino officer couldn't tell him, just before he turned the water cannon on a group nuns: "It's okay, I was trained at Fort Bragg."