Having won diplomatic victories against Vietnam in the U. N. General Assembly, Thailand and its Southeast Asian allies are now adopting a more conciliatory stance toward Hanoi in their efforts to convene an international conference on Cambodia.

Thai Premier Prem Tinsulanond, visiting China late last month, may have succeeded in slightly softening Peking's forceful posture against the cient government of Vietnam in Phnom Penh headed by Heng Samrin.

The pro-Chinese Khmer Rouge Communists were driven from the Cambodian capital almost two years ago by troops from neighboring Vietnam. Since then, a steady flow of weapons from China has helped Khmer Rouge guerrillas keep alive a bloody war of resistance.

Many analysts here see diplomatic maneuverings as a sideshow to this activity on Cambodian battlefields, where Vietnam's estimated 200,000 troops appear to have the upper hand. But Thailand, having obtained world censure of Hanoi, sees diplomacy as an important weapon. h

Last month Thailand helped push through two U. N. votes that kept the Khmer Rouge in Cambodia's General Assembly seat and called for an international conference to secure the withdrawal of foreign troops and bring about U. N.-supervised elections.

Also supporting these measures were Thailand's four fellow members of the Association of Southeast Asian Nations (ASEAN) -- Malaysia, Singapore, Indonesia and the Philippines.

Hanoi immediately condemned the conference proposal as interference in Cambodia's internal affairs, leading many diplomats here to question if the meeting would be convened at all. Without the presence of all parties to the conflict, there would seem little point in going ahead.

Moreover, Chinese intransigence against Vietnam seemed an almost equal threat to a conference. Having fought a month-long border war with Vietnam early in 1979, the Chinese Army continues to fight occasional artillery duels with Vietnamese forces across the common border.

In Peking, Thai Premier Prem appears to have argued for seeking solutions by all means, including negotiations. Voicing a common ASEAN stand, he reportedly said the cause of peace might be served if the current Khmer Rouge leaders were replaced.

Pol Pot, Eng Sary and Khieu Samphan are condemned as unrepentant mass murderers by most of the world community and the Cambodian people. But removing them might also satisfy the Vietnamese that their arch enemies were finished and this could prompt a reciprocal response. "The interest now is to show Hanoi that ASEAN is flexible," said one Bangkok diplomat.

Two personalities appear to be on ASEAN leaders' minds as alternatives. One is Son Sann, former premier and now head of the Khmer People's National Liberation Front, the most disciplined of the Khmer Serei (free Khmer) guerrilla groups.

Thailand has quietly nurtured the front's growth in reomote border areas since early 1979. This year the front's soldiers extended their control over two major refugee settlements in the border region, driving out rivals.

The other is Prince Norodom Sihanouk, head of state until 1970 and a thorn in the side of Thailand. Despite distaste for his personality and politics, Thai leaders now feel the prince could be useful in uniting ordinary Cambodians against the Vietnamese. For the present, however, Sihanouk has announced from his exile homes in China and North Korea that he has retired form politics. None of this, however, means that Thailand wants to "drop" the Khmer Rouge. Were it necessary, Thailand could neutralize much of the old regime's army single-handedly by denying it sanctuary and stopping the flow of Chinese arms across Thai territory.

Rather, the idea appears to be to deep the fighting force in the field to bleed the Vietnamese but to strengthen the resistance's hand by making its political leadership more acceptable -- both to the Cambodian people and Vietnam.

Just how the Thais would effect such a change is unclear. Pol Pot and Ieng Sary still command the loyalty of their troops, variously estimated at 25,000-50,000, and could be expected to resist any efforst toward their own disappearance. Any change they agreed to would by its nature be cosmetic.

Both Sihanouk and Son Sann, moreover, although lacking substantial armies of their own, have resisted concerted pressure from the Chinese to join the "united front" unveiled last year by the Khmer Rouge.

Returning from China, Prem told reporters that officials there had agreed with ASEAN that new leaders should be found -- a somewhat undiplomatic statement, given ASEAN's public stance that the Cambodian people alone can determine who governs them.

Statements by Chinese Vice-Foreign Minister Zhang Wenjin have further fed speculation that the Chinese are shifting their stance.

In the past, China has generally maintained in public statements that the Khmer Rouge are the legal government of Cambodia. However, Zhang simply said that China would not agree to any deal that completely eccluded the ousted leaders, implying that a coalition would be acceptable.

He also said that Cambodian resistance forces could benefit by uniting under someone other than Pol Pot, and, like Prem, suggested Sihanouk or Son Sann. Khmer Rouge leadership was not a precondition for Chinese support, he said, adding that his government did not agree that all support should be withdrawn from the Khmer Rouge.

China has heard a lot of the ASEAN analysis lately -- Singaporean Prime Minister Lee Kuan Yew arrived for a two-week stay shortly after Prem's departure -- and could now be signalling a significant shift in its position.

How such proposals will be received in Hanoi remains to be seen. But the foreign ministers of Malaysia and Indonesia are due to call in the Vietnamese capital soon and the question is certain to come up.