UN votes on Cambodia as hopes for peace rise. Vietnam-backed regime and rebel coalition to begin informal contacts

The United Nations General Assembly is expected to adopt a resolution today - for the ninth year in a row - demanding the withdrawal of the Vietnamese occupation force that drove into Cambodia (Kampuchea) in late 1978. But the resolution's adoption will, for the first time, take place amid a swirl of new developments that could help end the conflict between the Vietnam-backed regime and a rebel coalition fighting to overthrow it.

Some developments are clearly propaganda exercises intended to influence voting. Hanoi has been carrying on an intensive campaign to persuade governments that peace is just around the corner and must not be jeopardized by an unfriendly resolution.

Vietnam also has just announced a ``partial withdrawal'' of its troops. The annual exercise, suspected of being no more than troop rotation, would be carried out for the first time with ``foreign observers,'' Hanoi said.

More substantial developments include:

Hanoi and the Vietnam-installed Heng Samrin regime in Phnom Penh, have reached agreement in principle with the six-member Association of South-east Asian Nations (ASEAN) for informal ``cocktail party'' contacts among the war's four Cambodian factions.

Prince Norodom Sihanouk, president of the tripartite Coalition Government of Democratic Kampuchea (CGDK) battling to oust the Vietnamese, has opened the door to a meeting with President Samrin's foreign minister, Hun Sen.

China is backing off its support of the UN-recognized coalition's communist faction, the Khmer Rouge. As heir to the genocidal Pol Pot government that ruled from 1975-79, the Khmer Rouge has been an embarrassment to the coalition's supporters.

At the UN, the resolution vote drive was again steered by ASEAN's six members: Brunei, Indonesia, Malaysia, the Philippines, Singapore, and Thailand. Sponsors are concerned that they may lose a few votes this year as a result of the persuasiveness of Vietnam's peace campaign. But because of the lopsided advantage enjoyed by the resolution's supporters, including a record 63 co-sponsors, the resolution should pass easily.

Chinese sources here say Peking has shifted away from its past support of the Khmer Rouge. The new policy position was spelled out when China's foreign minister, Wu Xueqian, met his Soviet counterpart, Eduard Shevardnadze, in New York earlier in the current UN General Assembly session. Mr. Wu is said to have told Mr. Shevardnadze that China would not support a Khmer Rouge regime as the postwar government. There had been widespread concern that, because of its dominant military position within the coalition, the Khmer Rouge would take over when peace comes.

ASEAN sources suggest that there may also be a shift in the position of Moscow, until now the main supporter of Vietnam and, by extension, of the Samrin regime. As Singapore's ambassador, Kishore Mahbubani, pointed out, the Soviet bloc and Vietnam have ended a four-year boycott and are participating in the current general assembly debate, which opened yesterday.

The ambassador suggested that the development may be a spinoff of Soviet leader Mikhail Gorbachev's call for the strengthening of the United Nations. If that is the case, the Soviet Union would find it difficult not to support - or at least not to stop actively opposing - a resolution that has repeatedly had the landslide backing of the UN membership.