United Nations, N.Y. — The possibility of a thaw in Soviet-Chinese relations has introduced a new element into efforts to bring lasting peace to Indochina.
But despite a flurry of diplomatic activity aimed at Vietnamese-occupied Kampuchea (Cambodia), peace remains elusive.
The essential problem is that Vietnam is unlikely to withdraw from Kampuchea before a tacit triangular deal is struck between the Soviet Union, China, and the United States.
And although a dialogue of sorts has resumed between Moscow and Peking, an arrangement covering the highly sensitive dispute over Indochina is nowhere in sight, analysts here say.
Nonetheless, Vietnam and Thailand are described by well-placed diplomats here as worried about the implications of any Sino-Soviet thaw: ''Both countries want to make sure that their own big 'ally' (the Soviet Union and China, respectively) stays the course and that they will not be left twisting in the wind.''
Thailand fears that China's determination in keeping the Vietnamese out of Kampuchea may slacken and that as a result it would have to live with Vietnamese forces permanently stationed at its borders.'' Thailand's prime minister, Prem Tinsulanond, has scheduled a trip to Peking later this month.
Vietnam has reason to be concerned, because it has had to settle for less twice in its recent history: Once, in the 1954 Geneva conference with the French , the Soviets tacitly forced Vietnam to accept partition of the country into north and south; and in the early 1970s, both China and the Soviets supported President Nixon's request for a delayed takeover of South Vietnam by Hanoi while the US withdrew ''with honor.''
To settle for less now, Vietnam would have to live with an independent Kampuchea - independent of Hanoi as well as Peking.
''Peace is not in sight mainly because ancient, bitter national suspicions - Thailand vs. Vietnam, Vietnam vs. China - have been exacerbated by big-power jockeying for position [Soviet Union, China, US] in Southeast Asia. As each of the players wants to keep the pressure on, neither Cambodia nor Laos nor Vietnam is able to find peace and at the same time to remain independent,'' says a distinguished Southeast Asian diplomat.
Or, as another Indochina expert puts it: ''China, the Soviet Union, and the United States to some degree benefit from the present stalemate, and, despite official statements to the contrary, have no compelling interest to modify the present status quo.''
What do the superpowers gain from the present situation?
* China keeps Vietnam off balance and weakens it through support of the guerrilla war waged against the Vietnamese by the Khmer Rouge and the Nationalist Khmers led by Norodom Sihanouk.
* Moscow strengthens its dealings with Peking by having Vietnam act as a spearhead directed at China's underbelly. Also, thanks to Vietnam's dependency on the Soviet Union, the Soviets enjoy naval and air facilities in south Vietnam.
* The US, by supporting Thailand, China, and the Nationalist Khmers, keeps a foot in the area and good relations with China.
Meanwhile, the 35,000 Khmer combatants (22,000 Khmer Rouge and 13,000 Nationalists under Sihanouk's leadership) are not expected to expel Vietnam's 180,000 troops from Kampuchea. But neither can the Vietnamese finish off the rebels.
''If they attempted to do that, China would 'give them a second lesson,' '' a well-informed official says. He adds: ''In 1979 China launched an attack against Vietnam just as the Vietnamese were preparing to liquidate the Khmer resistance. China's offensive obliged the Vietnamese to divert troops from Cambodia to their northern border.''
Last week, the United Nations General Assembly voted by a wide margin (105 to 23 with 20 abstentions) in favor of a resolution asking Vietnam to pull its troops out of Kampuchea. There are no signs, however, that Vietnam intends to heed this call. Hanoi calls the situation in Kampuchea ''irreversible.'' It blames the impasse on China's desire to ''bring Vietnam to its knees.'' Meanwhile the Chinese continue to view Vietnam as an ''Asian Cuba'' contributing to Soviet encirclement of China.
''The Cambodian nut is very hard to crack because it pits against each other two deeply rooted, nationalistic ambitions. Vietnam believes that Laos and Cambodia should be under its influence, if not under its direct control. China expects the whole of Indochina, meaning Laos, Cambodia, and Vietnam, to pay tribute to itself, to be under its tutelage,'' says an old UN Indochina hand.
Hanoi calls on the countries of the Association of Southeast Asian Nations (Thailand, Malaysia, Singapore, the Philippines, and Indonesia) to discuss with itself all of Indochina's problems ''without exception'' and without preconditions. Vietnam's foreign minister, Nguyen Co Thach, recently visited Thailand, Singapore, and Malaysia and is now due in Manila and in Jakarta, where he intends to ''show Hanoi's goodwill.''
On a different track, UN Secretary-General Perez de Cuellar has pursued his own efforts to bridge the gap between the ASEAN countries and Vietnam. He dispatched a special envoy to Peking, Tokyo, Hanoi, Vientiane, and the five ASEAN capitals last February and then to Singapore, in June, to attend the ASEAN annual foreign ministerial meeting. He himself took up the Kampuchean problem with Soviet and Chinese leaders last summer during visits to Moscow and Peking. But reportedly he was not able to bring about any shifts of positions.
French Foreign Minister Claude Cheysson is expected to make a second trip to Hanoi in the near future. France, Sweden, and other Western nations believe that Vietnam would be happy to keep its distance from both China and the Soviet Union. The French hope to play a role in narrowing the gap between the ASEAN countries and Vietnam.
Romania's leader, Nicolae Ceaucescu, proposed setting up a ''national reconciliation government'' to include Vietnam-backed Kampuchean leader Heng Samrin as well as resistence leaders Sihanouk, Son Sann, and Khieu Samphan. Prince Sihanouk is expected to meet France's President Mitterand, then to visit Brussels, Bonn, and Ottawa. And Perez de Cuellar's special envoy is expected to visit Southeast Asia's capitals again in January.
The China-ASEAN alliance is based on ''sharing the same bed but not the same dreams,'' says a UN source. The ASEAN countries favor a neutral, genuinely independent Kampuchea. China does not seem to be willing to settle for a Kampuchea independent of Peking, according to the same source.
The pro-Vietnamese Laotian and Kampuchean authorities are not entirely pleased with the continuing presence of Vietnamese forces in their own countries , as they sometimes indicate in private talks.