Bangkok, Thailand — From the beginning, Vietnam's announcement that it would pull some troops out of Kampuchea (Cambodia) was greeted mostly by skepticism in Southeast Asia.
The noncommunist nations of the Association of Southeast Asian Nations (ASEAN) interpreted the July 7 announcement as an attempt to legitimize Vietnam's three-year occupation of Kampuchea. And so far, the goodwill tour of these countries by Vietnamese Foreign Minister Nguyen Co Thach has produced anything but good will.
Now, new evidence that has come to light suggests Vietnam's troop ''withdrawal'' is not a withdrawal at all, but actually a troop rotation.
Hanoi has not yet revealed how many of its estimated 160,000 to 180,000 soldiers in Kampuchea it plans to pull out. Diplomats in Hanoi guess at 20,000. Ho Chi Minh City newspapers say that 6,000 troops originally raised in the city have been demobilized since the beginning of the year, and hint that more are on the way. Not all of those demobolized, of course, will have served in Kampuchea.
A lot of troops stationed in Kampuchea should, however, be coming home soon. They were told that their stint of ''international duty'' would last four years, which means that those who came into Kampuchea with the original invasion force in December 1978 have almost finished their time.
But, judging from the enormous conscription exercise currently taking place throughout Vietnam, most will be replaced. Launched at the beginning of the year , the conscription exercise is divided into two phases. The first, completed in March, was described by the government as a great success.
The authorities have not given any nationwide figure for the number of recruits raised, but some idea of the magnitude of the operation is evident in the fact that just one of Vietnam's 36 provinces sent off 4,500 recruits at the end of the first enlistment drive. The second phase begins in a few weeks.
Many of the new recruits seem already to have arrived in Kampuchea, where they will complete their basic training. Some newly enlisted men were reported to have arrived in Kampuchea in February and March. More recently, however, the tempo of arrivals seems to have increased substantially. Reliable reports speak of large numbers of fresh troops - probably several thousand - passing through Phnom Penh.
The new reinforcements will apparently take over static defensive duties in areas largely free of Khmer Rouge activity, thus allowing troops with more combat experience to go after the new Kampuchean united front headed by Prince Norodom Sihanouk.
And the likelihood of the Vietnamese soldiers going out on a major, and perhaps politically sensitive, operation is growing. ''If the new coalition steps up its military activities,'' said a source close to Hanoi, ''we'll retaliate. The blow may come instantly, or in due course, but it will be hard.''
The partial ''withdrawal'' was announced by Mr. Thach during the Indochinese foreign ministers' meeting earlier this month, but the timing of the actual departure was clearly no coincidence.
The withdrawal, Vietnamese sources said, was intended as a goodwill gesture toward ASEAN - and as a reminder to the association that Hanoi has complete confidence in the stability of its allies in Phnom Penh. It was certainly not intended, they said, to suggest that Vietnam was loosening its grip on Kampuchea.
Thach originally described his visit to Singapore this week as a mission of ''peace, stability, cooperation, and friendship.'' But it quickly turned to recrimination and threat. On July 20, Thach accused ASEAN nations of carrying out hostile activities against Indochina and held out the possibility of an attack on guerrilla bases on the Thai border.
Sources said he was ready to compromise on secondary issues, but not on the heart of the matter - the continued existence in Phnom Penh of a government tightly allied to Vietnam. Singapore was not ready to compromise at all.
Thach will also be visiting Malaysia and Thailand, which, along with Singapore, are the ASEAN members most actively supporting the new anti-Hanoi coalition.