Since he was a boy in Saigon, Luke QuocThai Ngo wanted to be a jet pilot or a policeman.

"To me, the policeman is a symbol of a stand-tall, courageous, dignified person," said Ngo, 30, who has quit his job as an insurance underwriter to become Houston's first Vietnamese police cadet and one of a handful of Vietnamese law enforcement officials in the nation.

Ngo is making good on his boyhood dream not a moment too soon for the 50,000 South Vietnamese residents here.

Houston's Vietnamese community bustles with commercial vitality, but its peace has been marred by Vietnamese street gangs that have been preying on its people with little interference from police.

In the past two years, these gangs -- which, according to the President's Commission on Organized Crime, are part of nationwide Vietnamese crime syndicate operating in 13 states -- are suspected of committing half a dozen murders in Houston (none of which has been solved), as well as extortion, rape, robbery and burglary.

"Our people generally don't report these crimes to police," said Dr. Trinh Quang Vinh, a general practitioner whose office is in the heart of a neighborhood of Vietnamese restaurants, grocery stores, beauty salons, insurance agencies and other businesses just beyond the shadow of this city's gleaming skyscrapers. The shops seem rundown, but the Mercedes-Benzes outside testify to the hard work and relative affluence of this Vietnamese community, which in 1980 had an average family income of $25,926, according to Census Bureau figures.

"They have trouble making themselves understood, they're afraid of reprisals from the gangs, and they come from a country where the police were not always the most trustworthy," Vinh added.

Like many new immigrants, Vietnamese are easy pickings for criminals. Cultural barriers not only keep victims from reporting crimes; they also keep police from solving what few they hear about.

"We get reports about these gangs, but we're limited in what we can do," said Lt. Charles Cowan of the Houston-Harris County Organized Crime Control Unit. "Undercover work is virtually impossible. If we go out into any of these businesses, we stand out like the plague."

Police and community leaders here have been trying to keep the gangs from becoming entrenched. "We began meeting with the police several years ago, and the results are starting to show," said Nguyen Ngoc Linh, a former information minister of the South Vietnamese government. Linh publishes a Vietnamese-language newspaper here and heads a council of Vietnamese-American organizations.

"More and more, victims are willing to come forward. This summer we had some burglars tried and convicted and put in jail, which was a first. There is a trial scheduled . . . for three men suspected of murdering a prominent jeweler, and police now have mug shots of most of the gang leaders. The word is out that the heat is on. I would definitely say things have quieted down in the past few months."

Despite such progress, the community until now had not been able to get a Vietnamese onto the police force.

"Language has been the big problem," said Capt. Ephrine F. Leija, head of the department's recruiting division, which has screened out more than 100 applicants in its search for a Vietnamese officer. "Even those who speak English well have a tendency under stress to revert to Vietnamese. It could be a dangerous situation if you are radioing for help and can't make yourself understood."

There have been other obstacles as well. Police recruits must be citizens; only a small percentage of the half million Vietnamese refugees in this country have been naturalized.

Moreover, the age ceiling of 35 for police cadets here has disqualified most former military officers from applying. Meanwhile, the younger Vietnamese, those who attended high school here, are preparing for more profitable work. "They all want to be engineers," said Linh.

After two years of frustration, the community was delighted to get not one but three Vietnamese into the the current police academy class of 70 cadets. One dropped out in the second week, but now, five weeks into the 18-week training session, Ngo and Thong Anh Nguyen, 23, are going strong.

"Being the first is a half-and-half thing," said Ngo, a self-assured father of two who in 1972 was sent by his father, a professor in Saigon, to study at the University of Wisconsin. "You get your name in the history books, but there is a lot of pressure on you."

When they complete their training, Ngo and Nguyen will be assigned to regular beat patrol along with other rookies, but once they become more experienced, the department expects to use them to help combat the Vietnamese crime rings.

Nguyen is worried about working outside his community.

"I still have the feeling that I'm not a foreigner, I'm not American," said Nguyen, who went to high school and junior college here and became a citizen this fall. "I don't have blond hair and white skin," he continued. "If I am patrolling down the street and I stop somebody and issue a ticket, they might look at me, you know, and have the feeling that I might not carry the job right. So I probably feel more comfortable to work with the Vietnamese."

As Nguyen spoke of these misgivings in an interview, Ngo took the big-brother role.

"One point to help you with that is, what are Americans?" he said. "They come from everywhere. So don't feel that way. Don't worry about it. Hang on tight."

"I think this is a good profession," Nguyen said later. "Maybe in 10 years we'll both be captain of a unit of 10 or 20 Vietnamese policemen."

"Why just Vietnamese?" Ngo shot back. "You never know: In 10 or 15 years, maybe one of us will be chief of police."