Vietnam has applied a heavy dose of ceremony and diplomacy in recent weeks to promote its client regime in Phnom Penh headed by Heng Samrin.

The purpose, diplomatic observers here feel, is to help win it recognition as the legitimate government of Cambodia at the Havana conference of nonaligned nations, convening today, and later at the U.N. General Assembly in New York.

Still, observers here see few signs Vietnam will compromise on the most serious obstacle to recognition, the presence of Vietnamese troops in Cambodia.

Military analysts estimate 150,000 Vietnamese soldiers are deployed against the remnants of Pol Pot's Khmer Rouge army. Thai intelligence agencies claim two more Vietnamese divisions recently entered Cambodia's northeastern provinces, apparently in preparation for a new offensive when the monsoon rains end this fall.

Despite their alleged record of atrocities against their own people, the Khmer Rouge are still recognized by most countries as Cambodia's legitimate government. Vietnam apparently hopes that if the Khmer Rouge are isolated diplomatically, aid to their army -- primarily from China -- might taper off.

Heng Samrin's administration was installed by Vietnamese-led troops after a lightning invasion eight months ago. But so far it has been recognized only by Soviet-bloc countries and a handful of others. Analysts here see several developments as Vietnamese-inspired efforst to enhance that government's international standing.

Heng Samrin, for example, has begun making "state visits" abroad. Last month the Khmer leader, believed to have been a low-ranking officer in Pol Pot's army before joining with the Vietnamese, visited Laos, another Vietnamese ally. There he was accorded a reception fit for a head of state.

Next came three days in Hanoi, where he was feted by Vietnamese Prime Minister Pham Van Dong at what Radio Phnom Penh described as a "grand reception."

From Hanoi the party left for Havana, with a stopover in Moscow. Now it is awaiting a decision by the nonaligned conference on which faction can represent Cambodia.

Heng Samrin's attempts to become respectable by rubbing shoulders with recognized leaders is being mimicked by the Khmer Rouge. Their four years in power were characterized by scorning of things and people foreign. Today, however, their radio station notes their every official contact with foreign leaders.

Recently, for instance, Khmer Rouge broadcasts noted that head of state Khieu Samphan had sent messages to the kings of Spain and Belgium, congratulating them on their countries' national days. Both monarchs duly thanked him, the radio noted.

When Khieu Samphan and Khmer Rouge Deputy Prime Minister Ieng Sary flew to Havana last week they stopped over in Vienna. There, the radio reported, they were "warmly welcomed" by Austrian protocol officials and spoke to "many journalists and reporters."

The trial in absentia of Pol Pot and Ieng Sary has been part of the new regime's campaign for legitimacy. Phnom Penh convened legal proceedings against the two leaders last month on charges of genocide. The trial was presided over by the Heng Samrin government's minister of information, press and propaganda, and was much publicized by Communist media.

Witnesses recounted stories of mass murder. One women said 45 members of her family died during the Khmer Rouge's rule. A man testified that the Khmer Rouge authorities ordered the execution of 2,005 people to celebate their 1975 victory. Foreign lawyers, including two Americans, testified that the proceedings were in accordance with international law.

The defendant were found guilty of causing the deaths of 3 million Cambodians and were sentenced to death. Observers here feel the trial's purpose was to cast the two men internationally as convicted criminals, not deposted cabinet ministers.

The Heng Samrain faction is insisting that any foreign famine relief pass through its hands. International agencies have been unable to secure permission to supply hungry Cambodian civilians directly from Thailand. The Heng Samrin government requires that aid be flown to airfields under its control and distributed by its agencies, leading to fears that some food might be diverted to the Vietnamese army.

Diplomats here believe Phnom Penh's motive is partly to obtain further standing for itself by forcing foreigners to deal directly with its officials.

Last month, Thai sources report, Soviet and Romanian diplomats informed the Hair mission in Hanoi that Vietnam would pull out a small number of its soldiers by the end of the year. The Thai government appears to give little credence to the report, however, and thinks it was meant by Hanoi to be a goodwill gesture.

Though the success in Havana of all these efforst to press Heng Samrin's legitimacy remains to be seen, they had had little effect on Cambodia's immediate neighbors. The five members of the Association of Southeast Asian Nations are firm in insisting the Khmer Rouge are the only legitimate government of the country.

When Heng Samrin visited Hanoi, he and his hosts announced that all Vietnamese troops would leave Cambodia as soon as China (it was not specifically named) stopped its "aggression against the independence, territorial integrity and peace" of Cambodia. China, meanwhile, has affirmed it has no intention of ending aid to the Khmer Rouge.

Given these facts, and Vietnam's long-proven capacity for marathon warfare, most analysts here expect heavy fighting will resume in Cambodia when the monsoon rains end.