Facing each other across a roadblock near a captured Cambodian guerrilla camp, Vietnamese and Thai soldiers today argued about where the Thai-Cambodian border lies and warned each other against crossing it.

The unusual incident at the main western entrance to the Ampil Cambodian resistance settlement just across the border from the Thai village of Ban Sangae occurred a day after Vietnamese forces overran the camp, forcing guerrillas of the anticommunist Khmer People's National Liberation Front to flee into Thailand.

While some skirmishing between guerrillas and Vietnamese troops was reported in the area, the appearance of the relaxed and unarmed Vietnamese soldiers at the roadblock next to an antitank ditch several hundred yards inside Thailand indicated that the invaders have the run of Ampil.

The incident, filmed by a Thai cameraman, came as Thailand lodged a protest against Vietnam at the United Nations in New York over the shooting down yesterday of a Thai Air Force plane and two alleged border incursions by Vietnamese troops.

The Thai government also announced it was boycotting a week-long meeting of the Interim Mekong Committee due to begin Thursday in Hanoi. A statement said the boycott was to protest Vietnamese violations of Thai sovereignty that caused the deaths of several Thai soldiers and damage to civilian property. The U.N.-sponsored committee meets periodically to discuss Mekong River development projects.

A more forceful statement was issued today by China, the main backer of the Cambodian resistance groups battling the six-year-old Vietnamese occupation of Cambodia. A Foreign Ministry spokesman in Peking denounced the current Vietnamese dry-season offensive along the Thai-Cambodian border and demanded that Vietnamese troops stop committing "atrocities" and "slaughtering" the Cambodian people.

The statement also rejected Hanoi's recent offer of a seven-week truce along the Sino-Vietnamese border to mark the Lunar New Year.

China and Vietnam fought a brief war along their common border in 1979 shortly after the Vietnamese invasion of Cambodia. Since then, Sino-Vietnamese clashes have repeatedly coincided with Vietnamese offensives against Cambodian guerrillas.

Strongly worded as they were, the diplomatic protests contrasted oddly with the scene on the Thai-Cambodian border today when a Thai sergeant and a couple of Vietnamese soldiers engaged in their own border dispute, relying mostly on gestures since they could not speak each other's languages.

After the Vietnamese sauntered unarmed up to the roadblock, the Thai sergeant warned them that they and half a dozen comrades a short distance behind were on Thai soil and had better move back at least 300 meters from the antitank ditch.

The Vietnamese squad leader, wearing a cap with a red star on it, argued with sweeping gestures that the border came up to the roadblock and that the Thai soldiers should stay behind it.

Later, a Thai officer came to the antitank ditch and warned the Vietnamese in their language through a loudspeaker to withdraw from Thai territory or face attack. Thai authorities said the Vietnamese complied, at least initially.

The routed Cambodian defenders of Ampil, meanwhile, were regrouping for a counterattack, according to Col. Chettha Tharnatho, the deputy commander of the Thai eastern border task force.

He said guerrillas from two other Khmer front camps overrun earlier by the Vietnamese were on their way to join the Ampil guerrillas.

A Vietnamese attack on the remaining resistance settlement, at Dong Ruk about four miles north of Ampil, is expected at any time, and its 17,500 civilian population is preparing to evacuate, western relief officials said.