After two weeks in Vietnam and Kampuchea, four retired US diplomats have concluded that America's Indochina policy is one of ''shrinking assets'' - the Vietnamese are not budging from their occupation of Kampuchea, and the Khmers seem to prize above all else the stability that the Heng Samrin regime has given them.
In place of the present US policy of recognizing the anti-Vietnamese coalition government of Democratic Kampuchea and supporting its representation at the United Nations, some of the group suggest a gradual move toward leaving empty Kampuchea's UN seat.
The four diplomats include two ambassadors: James Leonard, deputy chief of the US mission to the UN in the late 1970s, and Emory Swank, US envoy to Phnom Penh from 1970 to 1973, the first years of the war in Cambodia; and two Asia hands, Paul Kattenburg, now professor of political science at the University of South Carolina, and Don Ranard, now director of the Center for International Policy, the Washington-based group that organized the trip.
Starting in Hanoi, the diplomats visited Ho Chi Minh City (formerly Saigon), Phnom Penh, the eastern Kampuchean (Cambodian) province of Prey Veng, and the temples of Angkor Wat.
The group seemed perhaps most struck by the psychological scars left on the Khmers by the Pol Pot years, when hundreds of thousands, if not millions of people died from starvation or neglect or were executed.
''You have to be exposed to their stories firsthand to realize that they are deathly afraid of any perturbation that might lead to a return of Pol Pot,'' said one of the group.
''They have had instability for so long that stability means everything to them - it means, for example, the ability to live together as a family, something they were not able to do under Pol Pot,'' he continued.
''And to produce a baby every 10 months,'' another added, referring to Kampuchea's estimated 6.5 percent birthrate, probably the highest in the world.
Vietnamese soldiers in Kampuchea, the group said, were behaving themselves ''extremely carefully'' and seemed to be well accepted by the Khmers. Western reports from Afghanistan, one of the diplomats noted, describe Soviet troops moving around Kabul ''not in pairs, but in companies.'' In Kampuchea ''you see them shopping in markets or traveling along roads alone and unarmed,'' he said.
Despite longstanding Khmer-Vietnamese animosity, the diplomats felt that most Khmers accepted the Vietnamese as an essential line of defense against Pol Pot and the Khmer Rouge. A senior member of the group said that any Vietnamese military disengagement would be a delicate operation: ''To avoid panic the Vietnamese would have to withdraw over, say, a five-year period,'' he suggested.
The Heng Samrin regime, meanwhile, has not proven to be an onerous socialist regime. ''Some officials told us they had introduced taxes at the beginning of the year,'' one said, ''but no one could explain how they worked.''
In private chats with Khmers, the visitors found little interest in Son Sann, one of the three leaders of the anti-Vietnamese Coalition Government of Democratic Kampuchea (CGDK), but considerable resentment for the other noncommunist leader, Prince Norodom Sihanouk.
Many of the people they spoke to were survivors of the middle-class intellectuals who had opposed Sihanouk in the 1960s and ''who had not forgotten the help Sihanouk gave Pol Pot in recruiting peasants for the Khmer Rouge,'' after his overthrow in 1970, said one of the Americans.
Phnom Penh had no intention of broadening the government by inclusion of ''reactionaries like Pol Pot, Son Sann, and Sihanouk,'' Kampuchean foreign minister Hun Sen declared. Referring to his statement of last Sept. 18, which offered amnesty and full civic rights to any Khmer who abandoned the CGDK, Hun Sen pointed out that he did not name Sihanouk - ''although you can infer it.'' The Vietnamese, who clearly feel more warmly toward the prince than their Khmer allies, certainly do infer this and have said so on a number of occasions.
Hun Sen added that the amnesty offer had a definite time limit. If ''reactionaries'' like Sihanouk surrender while they still have some forces, he said, they will receive clemency. If they wait until their forces have dwindled to nothing and then request an amnesty, they will ''be treated as prisoners of war.''
The foreign minister also had tough words to say about the refugee camps controlled by the anti-Vietnamese coalition government along the Thai-Kampuchean border.
Referring to Nong Chan Camp, which was overrun by Vietnamese troops while the American group was in Vietnam, Hun Sen asked rhetorically, ''Is this a refugee camp or a sanctuary for reactionaries?'' His government, he added, ''reserved the right to destroy reactionaries.''
Hun Sen's counterpart in Hanoi, Foreign Minister Nguyen Co Thach, was less menacing but equally unbending. ''We got the impression that the Vietnamese will stick to their present position forever,'' said one of the group. Thach, the diplomats said, was ''hard and theatrical.'' Like some of their other Vietnamese interlocutors he was welcoming but slightly wary, apparently having difficulty accepting that the Americans were only private citizens.
The Americans raised the issue of reeducation camps several times and expressed dissatisfaction at the answers they received, which they describe as ''orchestrated.'' They were told that there are only 10,000 people left in the camps: Western sources have estimated from 20,000 to 60,000. They were also denied permission to visit a camp, although such visits have been granted to other visitors.
Mr. Thach was one of those who took the hard-line, saying essentially, according to one of the group, that ''90 percent of the people in reeducation were guilty of capital offenses, and that the government was therefore showing clemency by not putting them on trial.'' When one of the Americans suggested an international commission to examine the issue, his interlocutor retorted that this would be ''interference in Vietnam's internal affairs.''
But Minister of Justice Phan Hien told the group that the government was considering releasing the detainees into the custody of local women's and youth union chapters for ''reeducation in place'' in their home areas.
Though unhappy with reeducation, the group was skeptical of the claims made Feb. 8 by the State Department that Vietnam is the worst place in the world to live in terms of human rights. ''A political judgment, I think,'' said one.
''I didn't see anything to justify that, and I'm shocked that they should say Vietnam is worse than the Soviet Union,'' said another.
The diplomats were impressed by the caliber of some of their hosts. Mr. Thach and Phan Hien would, one of them said, ''get promoted in our system.'' But they were also struck by the absence of Americanologists in Vietnam. ''They have people who speak beautiful Chinese and are getting good Russian linguists,'' said one of the Americans, ''but they seem to have given up on us.''
The US policy of diplomatic isolation and economic pressure on Vietnam was, the group felt, not working, and its Kampuchean policy was achieving effects opposite to those sought. ''Our present policy is trying to modify the nature of the Phnom Penh government,'' one of the group remarked, ''but because of isolation the government is turning into another Vietnam. Khmers are being trained in Eastern Europe and Vietnam, and there are no mitigating factors - the US is unable to offer any alternatives.''
What members of the group would like to see is a ''measured move away from the present policy.''
''We ought quietly to urge ASEAN (the Association of Southeast Asian Nations) to move gently toward an empty seat in the United Nations,'' said one of the former ambassadors. This change in policy should happen maybe one or two years from now.
After a visit to a Son Sann-controlled refugee camp on the Thai-Kampuchean border, one of the diplomats remarked sadly, ''I feel history repeating itself. Here we are supporting a group of Khmers who have even less hope of success than Lon Nol did.''