Vietnamese civilians are moving into Cambodia in growing numbers under a new policy designed to encourage settlement of that country, already occupied by as many as 180,000 Vietnamese troops, according to western diplomatic and academic sources.

The policy is explained in two documents issued last fall by the Vietnamese-installed government in Phnom Penh and smuggled to the Thai-Cambodian border, apparently by dissidents who oppose the Vietnamese settlement. Recent visitors to Phnom Penh and Hanoi have confirmed that the documents are authentic and that the resettlement was under way.

Analysts here differ as to whether the policy is part of a long-range Vietnamese plan to dominate Cambodia or a short-range measure to help both countries economically. But the unusual documents seem certain to fuel a charge repeatedly leveled by Vietnam's opponents: that Hanoi seeks in effect to "colonize" Cambodia. At the least, the documents indicate that Vietnam and its protege government in Phnom Penh led by Heng Samrin consider the settlement issue politically sensitive.

The circulars, dated Sept. 13 and Oct. 9 last year, acknowledge that the issue of Vietnamese living in Cambodia is "complicated and easy for the enemy to inflame." They warn that the opposition could "sow panic by psychological warfare to divide the two nations" and call for efforts to eliminate "narrow-minded nationalism" among Cambodians and Vietnamese.

According to the Oct. 9 circular signed by the Phnom Penh government's prime minister, Chan Si, about 500,000 Vietnamese lived in Cambodia before 1969. The document said their lot worsened when the Lon Nol government came to power in 1970. It said the Vietnamese suffered torture and execution when the communist Khmer Rouge led by Pol Pot took over in 1975.

"Those who survived are very few," the circular said.

It went on to instruct "all ministries, state institutions at the center and people's revolutionary committees of each province and municipality" to "assist and support" those surviving Vietnamese who have "returned to Kampuchea Cambodia to live and work honestly."

In addition, the document called on authorities to help "Vietnamese people who have come following the day of liberation"--an apparent reference to the January 1979 ouster of the Khmer Rouge by Vietnamese forces--and who are involved in "work to rectify and expand the economy such as farming, lumbering, fishing, salt working and handicrafts."

It said permission should also be given for friends and relatives of resident Vietnamese to join them in Cambodia.

The circular indicated that Vietnamese "elements who betray the revolution or make illegal livings by infringing on state laws" should be dealt with by Vietnamese advisers. It concluded by calling on Cambodian authorities to work with the Vietnamese advisers toward "expanding rapidly the movement of the Vietnamese people."

According to a U.S. diplomat here who monitors Cambodia, the new policy "is clearly facilitating and encouraging Vietnamese immigration" far beyond the return of former residents.

The diplomat cited the account of an elderly fisherman who said there were more Vietnamese fishermen around the Tonle Sap lake now than at any other time in his life. Refugees also have reported the movement of Vietnamese traders and their families as far west as Battambang, most of them new settlers rather than former residents, the diplomat said.

He said Vietnamese farmers have settled on fertile river bottom land in Takeo Province for the first time. In some cases, refugees have claimed that Cambodians were not permitted to fish in areas settled by the Vietnamese.

"Basically the Cambodians have to move out of the way," the diplomat said. "Refugees at the border cite the Vietnamese as a reason for leaving. Not only Vietnamese control but the presence of Vietnamese."

How many Vietnamese settlers currently live in Cambodia remains uncertain. The Bangkok English-language daily, The Nation Review, recently quoted "intelligence reports" as putting the figure at 300,000, with half of them having flowed in since February 1982.

The U.S. diplomat said that the Vietnamese could number 150,000 to 200,000 but that this was still a guess.

"My view is it's a clear indication that the Vietnamese have left the option open of absorbing Cambodia if the Heng Samrin regime can't get its act together," the diplomat said.

That view is disputed by William Turley, a visiting American professor at Bangkok's Chulalongkorn University who recently traveled to Hanoi and discussed the issue at length with a senior Foreign Ministry official. He said the official, whom he named, requested anonymity in any articles about the trip.

Turley said the official summarized the contents of the documents issued by the Phnom Penh government, confirming that the policy they described does exist. In a three-hour meeting March 31, Turley said, the official indirectly acknowledged difficulties with traditional Vietnamese-Cambodian ethnic antagonism by noting that "history has left a problem that can be exploited by our enemies."

He added, according to Turley, "It is normal for Vietnamese to want to live in Cambodia, but their treatment has not been good." The official said Hanoi sought to eliminate discrimination so that Vietnamese in Cambodia would be treated the same as Cambodians.

The senior Foreign Ministry official said that while former Vietnamese residents of Cambodia would be allowed to return freely, "others must seek permission, and these others are subject to limits," Turley said. He said the official did not elaborate.

Turley said he believed the main reason for the settlement policy was to develop the Cambodian economy. He said this development "depends on the presence of people who can perform certain economic functions" and that Cambodia lacks these people. Ironically, this means that settlers from communist Vietnam are being encouraged to engage in free-market activity in Cambodia--within authorized limits--to help get the country on its feet. The policy also may help to relieve unemployment in Vietnam, Turley said.