Western diplomats here see an offer by Vietnam this week to withdraw troops from Cambodia as a propaganda gesture right before a meeting of nonaligned nations, but western communists here assert that the offer, plus a qualified promise to pull out more troops annually, could be a step toward rapprochement with China.

Vietnam made the offer to withdraw an undisclosed number of troops in news conferences and press releases here and in several other capitals Wednesday.

Foreign Minister Nguyen Co Thach, conducting the press conference here, also appeared to confirm reports that a squad of former American Special Forces soldiers had infiltrated Laos to search for U.S. prisoners of war they believe are still being held 10 years after U.S. troops withdrew from Vietnam.

Terming this "a flagrant violation of the sovereignty of Laos," Thach repeated Vietnam's longstanding denial that any missing Americans are left in Vietnam, Laos or Cambodia.

An armed team of ex-Green Berets and Laotian anticommunists led by retired Special Forces lieutenant colonel James G. (Bo) Gritz reportedly has entered Laos searching for Americans. Vietnamese and Laotian troops are said to be hunting Gritz's group, and Thai police are waiting along the Mekong River border to arrest them if they cross back.

In documents marking the end of a two-day meeting of the leaders of Vietnam and its allies Cambodia and Laos, held in the Laotian capital of Vientiane, Hanoi pledged to withdraw some of its troops from Cambodia in 1983.

U.S. and other western officials dismissed a similar announcement last year, charging that Vietnam was actually rotating troops.

The latest Vietnamese statement said that "some more units" of Vietnamese troops, which were called "volunteers" for the first time, would be pulled out in 1983. In addition it said that "each year a partial withdrawal of volunteers from the Vietnamese Army . . . will be decided upon with due consideration for the security" of Cambodia.

Concerning numbers of withdrawn troops this year and last, Thach said, "I am a diplomat and therefore I can't let you know something the men from the Army don't let me know."

"A lot of it is cosmetic," one Western diplomat said. "It's interesting that this should happen before the nonaligned meeting."

Sharp debate is expected at preliminary sessions next week over who should represent Cambodia at the March 7-11 meeting in New Delhi: the Heng Samrin government installed by Vietnam in January 1979 or a U.N.-recognized coalition of resistance groups nominally headed by Prince Norodom Sihanouk.

India, the only noncommunist country to recognize the Heng Samrin government, has refused as host to invite Sihanouk.

The Vietnamese "can't go on having bogus partial withdrawals year after year," said a western embassy official. A contrary view, advanced by western communists here, was that the Vietnamese offer could be a response to a reported shift in the conditions of Hanoi's archenemy, China, for discussing normal relations.

Chinese officials reportedly told two French delegations in December and January that Peking would negotiate with Vietnam once Hanoi put forward a timetable for withdrawing its troops from Cambodia. Previously, China had stipulated a complete Vietnamese withdrawal as a prerequisite for such talks.

Nevertheless, the diplomats agreed that no sign of flexibility emerged from either the Vientiane summit or the press conference here on the crucial question of who should govern Cambodia.

At his news conference, Thach ruled out any elections involving members of the ousted communist Khmer Rouge government, which ruled Cambodia brutally for nearly four years before the Vietnamese invaded. Nor, he said, would "those who collaborate directly or indirectly" with the Khmer Rouge be allowed to participate. Thach's statements apparently excluded involvement of Sihanouk and another noncommunist resistance leader, Son Sann, president and prime minister respectively in the coalition with the Khmer Rouge.