Phnom Penh, Kampuchea — There was a conspicuous absentee from the 40-odd foreign journalists who endured last week's Vietnamese troop withdrawal from Phnom Penh, Kampuchea. It was the correspondent of Xinhua, China's official news agency.
In one way, of course, the absence was hardly surprising, as Vietnam and China had been in a state of near-war for four years and had just just finished trading shots across their border.
The surprising thing was that Xinhua had been invited at all, and Vietnamese officials were careful to express regret that no one from Xinhua had been able to make it.
Peking's official response to the pullout was to brand it a fake - simply another troop rotation. In private, though, Chinese diplomats in Southeast Asia have been less dismissive. Well-informed diplomatic sources in the region say the Chinese have been careful in their comments on the withdrawal, but certainly not negative.
One of them, for example, reportedly took care to say that he did not believe the withdrawal was genuine - but that if it was, it would be a step in the right direction. And, the official is said to have continued, if 20,000 troops had been pulled out - as some diplomats have speculated - this would constitute a significant withdrawal. Another Chinese diplomat reportedly went further: A genuine partial withdrawal, he said, might lead to Sino-Vietnamese talks.
The officials who made these remarks were senior enough to carry some weight, but not senior enough to formulate policy. As Chinese diplomats are seldom given to voicing personal opinions to foreign colleagues, however, their remarks seem to provide an initial idea of how Peking is viewing the changes.
Last year's announcement of a Vietnamese pullout was greeted with a generous degree of skepticism - in large part due to reports that busloads of fresh Vietnamese recruits were arriving in Phnom Penh from Vietnam while the supposed withdrawal was taking place. This year there are compelling reasons to take a withdrawal claim more seriously:
* Successes on the battlefield. In late January Vietnamese troops took, with little difficulty, the large camp run by Son Sann, one of the three leaders of the anti-Vietnamese Coalition Government of Democratic Kampuchea. Two months later Vietnamese troops overran a Khmer Rouge divisional headquarters at Phnom Chat and Prince Sihanouk's headquarters - just about his only base - at a camp called Sihanoukville. The poor showing of the prince's men was expected; the disastrous showing of the Khmer Rouge was not.
Western reports talk of the Vietnamese removing six truckloads of military supplies from the captured Khmer Rouge camp. The Vietnamese say they captured 20 tons of ammunition and explosives.
The Chinese, who had supplied all of the materiel captured by the Vietnamese, were quietly dismayed. ''The Khmer Rouge can no longer take on large Vietnamese units,'' a Peking diplomat commented afterward. ''They will have to rely on guerrilla warfare.''
* Possible hopeful diplomatic signs. During the nonaligned conference in New Delhi in March, Vietnamese Foreign Minister Nguen Co Thach and his Malaysian counterpart, Tan Sri Ghazali Shafie, had a chat. In the course of the conversation they agreed on what is now known as the five plus two formula: talks between the five countries of the Association of Southeast Asian Nations (ASEAN) and two countries of Indochina - Vietnam and Laos, but excluding Kampuchea for the time being.
Vietnam's Khmer allies were not amused by the proposal and seem to have taken some time to be brought around to accepting it. Now they accept the formula, even though Kampuchean Foreign Minister Hun Sen pointedly called it a ''sacrifice'' on the part of his country when he spoke to journalists in Phnom Penh on the eve of the troop withdrawal.
The pullout was presented to journalists in Phnom Penh as a gesture of goodwill to ASEAN, and as a sign of faith in the security situation and the combat abilities of the new army of the People's Republic of Kampuchea. The army now numbers around 33,000 men.
The Vietnamese garrison in Kampuchea before the pullout was generally estimated at 160,000 to 180,000 men. The diplomatic estimate of 20,000 departing troops seems a little high. The Vietnamese, with their usual urge for secrecy, said only that it was ''certainly more than 10,000, a figure of 12-14,000 is probably a resonable guess.''
Conversations with soldiers - strictly forbidden by the Vietnamese authorities - give a picture of a battle-hardened outfit. The commander, Col. Vo Van Dan, is a southerner and a veteran of the war against the French. It was in that war that he first fought along the Khmer border, according to a former comrade.
The noncommissioned officers, most of whom were in their late 20s, had taken part in the last assault on Saigon (now Ho Chi Minh City), almost eight years to the day before they left Phnom Penh. Most of the privates were conscripted in 1978 and saw combat almost immediately along the Kampuchean border, before becoming some of the first troops to enter Phnom Penh in January 1979. They followed the retreating Pol Pot forces - ''all over the place'' said a soldier with a broad sweep of his arm - ending up on the Thai border. In the last year these Vietnamese soldiers have had a change of assignment: They trained one of the new Kampuchean army divisions.
The troops are a mixed bunch, southerners and northerners, farm boys and factory workers. They hope to be demobilized when they get home to Vietnam, but are not sure. And then? One plans to go back to his village ''and not go anywhere else ever again.''
Another said he wanted ''to go to a socialist community in the country and learn to be a fitter.'' He said that would be fun.