United Nations, N.Y. — Vietnamese Foreign Minister Nguyen Co Tach shows no interest in a new proposal for United Nations supervision of Kampuchea's western-most province. The proposal calls for Vietnamese troops in Kampuchea to pull back from the province as a token of Hanoi's sincerity. Presumably, resistance forces under the Khmer Rouge would be prevented from moving into these areas.
The offer came last week from the Association of Southeast Asian Nations (ASEAN), or Malaysia, Singapore, Thailand, Indonesia, and the Philippines.
''The UN recognizes the Khmer Rouge as the legitimate government of Kampuchea - how can we trust the UN to stop them from capturing these territories?'' Foreign Minister Tach asks, in an interview with the Monitor.
Only if China agrees to restrain the Khmer Rouge could this idea become acceptable to Hanoi, but the nod from Peking has not come, according to him. Tach hails the new ''realistic'' tone of ASEAN toward Vietnam but sees the proposal as ''soft on words, tough on substance.''
Cambodian ex-Prince Norodom Sihanouk, heading a coalition of resistance groups, says the proposal puts Hanoi on the defensive, ''because it shows that we, not they, are reasonable and humane.
''Time is on Vietnam's side. The Vietnamese have shown . . . they are willing to make incredible sacrifices.''
The war waged against the Vietnamese is one way to keep Hanoi off balance, says Sihanouk, but he does not think they can be pushed out of Kampuchea.
Tach says ASEAN is moving away from China's anti-Hanoi position, and believes a thaw in Sino-Soviet ties can benefit Vietnam. And he is satisfied that several West European nations are moving toward improved ties with Hanoi.