United Nations, N.Y. — Vietnam is in a ''conciliatory mood'' about the future of Kampuchea (Cambodia), says Foreign Minister Nguyen Co Thach. He says that:
* Vietnam has already removed some of its forces from Kampuchea and plans to withdraw more.
* It is seeking to engage in discussions with the United States government regarding the resettlement in the US of 8,000 Vietnamese dissidents confined in reeducation camps.
* Another US mission will be allowed to visit Vietnam within two months to discuss US soldiers missing in action (MIA).
In an interview here, Mr. Thach said was hoping to meet with US officials during his stay at the United Nations to discuss bilateral issues and to ''clear the atmosphere.''
''The US has a constructive role to play in helping bring peace and stability to Southeast Asia,'' he said.
One ambassador from the six-member Association of Southeast Asian Nations (ASEAN) compared Nguyen Co Thach to a wild cat. ''He purrs but beware of his claws. Every year, before the UN General Assembly votes to decide on who - Democratic Kampuchea or the Heng Samrin regime - shall sit in Kampuchea's chair, he softens his tune.''
At present Kampuchea is represented in the UN by a tripartite Coalition Government of Democratic Kampuchea. The anti-Vietnamese coalition is led by Prince Norodom Sihanouk as president, Son Sann as prime minister, and Khieu Samphan, leader of the communist Khmer Rouge, as vice-president.
Close to 100 countries at this UN are expected once again to reject the legitimacy of Heng Samrin, who was installed by the Vietnamese in 1979. ''But with the passing of time that UN majority may slowly erode,'' a nonaligned diplomat believes.
Southeast Asian sources say that every time Vietnamese troops leave the Kampuchean capital of Phnom Penh for Ho Chi Minh City in southern Vietnam in front of foreign observers, fresh Vietnamese troops are brought in along other roads with no observers watching.
While it is true that Indonesia's foreign minister plans to meet with Thach at the UN, and while Indonesia does enjoy friendly bilateral relations with Vietnam, this meeting can in no way be considered a preliminary phase of an ASEAN-Indochinese conference, ASEAN sources say. They insist that ASEAN remains united in opposing Vietnam's occupation of Kampuchea.
US sources claim that there are no plans for high US officials to meet with Thach at the UN.But American officials have recently expressed interest in Vietnam's show of goodwill.