Bangkok — About 10,000 Vietnamese troops are returning home this week from Kampuchea, according to official Vietnamese reports. The partial troop withdrawal, the third in three years, was another effort by Hanoi to demonstrate its confidence in the state of security of its Kampuchean (Cambodian) ally.
The troops are said to have been stationed in the western and northwestern provinces of Pursat, Battambang, and Siem Reap. All three provinces have extensive borders with Thailand, and have been the scene of much of the fighting between the Vietnamese and its Khmer allies on one side, and the Western-backed Coalition Government of Democratic Kampuchea (CGDK) on the other.
Official announcements of the pullout, made virtually simultaneously in Hanoi and Phnom Penh on June 15, portrayed the withdrawal as a sign of strength.
The Phnom Penh communique, for example, claimed that the pullout followed six months of major victories in the fight against the CGDK. In particular, Phnom Penh claimed to have ''put out of action'' over 4,500 enemy soldiers. The figure is significant: In previous years neither Phnom Penh nor Hanoi would probably have admitted that it faced an emeny whose total armed strength amounted to this much.
One of the farewell ceremonies touched on a subject that is usually taboo in Indochina: Vietnamese losses in the fighting against the CGDK. Speaking to departing troops in the western province of Battambang, the province party chief , Koy Buntha, noted ''with sadness'' that ''many'' Vietnamese troops would never return home, ''having laid down their lives for the new life of the Kampuchean people.''
In the past, the Vietnamese had tended to stress the insignificance of combat against the coalition forces. Koy Buntha's comments apparently underline the new tendency of Hanoi and its Khmer allies to admit that serious fighting is taking place.
The withdrawal, if genuine, leaves about 150,000 to 160,000 Vietnamese troops in Kampuchea.
This is probably enough to contain the insurgency but not to defeat it. Conventional military wisdom dictates that a government wishing to defeat a guerrilla movement should have a numerical advantage of 10 to one in combat troops.
Only half the remaining Vietnamese troops are fighting units. The others are support personnel. The CGDK, on the other hand, is estimated to have 45,000 to 50,000 armed men, the majority of them Khmer Rouge. The current ratio of defending soldiers to guerrillas, therefore, is down to less than two to one.
Members of the CGDK, as well as some of their supporters, the Association of Southeast Asian Nations and Peking, have dismissed the pullout as simply a disguised troop rotation. A Peking commentary on the subject was entitled ''The same old stuff.''
The Khmer Rouge, however, claim to have information that 3,000 fresh Vietnamese troops moved into Kampuchea just before the official withdrawal. Khmer Rouge radio claimed on June 25 that the new troops were now stationed in Battambang.
Khmer Rouge sources are notoriously unreliable.
But while last year's Vietnamese pullout was generally held to have been genuine, the 1982 pullout apparently coincided with an infusion of fresh military recruits.