Vietnam has informed the United Nations that it is ready to withdraw some of its forces from Cambodia if a Vietnamese-sponsored "demilitarized zone" is established to stabilize the troubled Cambodian-Thailand border, informed sources said today.

The offer was conveyed to Secretary General Kurt Waldheim on Tuesday by Vietnamese Foreign Minister Nguyen Co Thach, who is here for the opening sessions of the U.N. General Assembly. Thach did not specify how many of the 200,000 Vietnamese troops would be withdrawn under this plan, according to the sources.

Such a limited withdrawal of Vietnamese troops under this arrangement appeared unlikely to be realized because Thailand has flatly rejected the Vietnamese demilitarized zone proposal, announced last July 18 in Vientiane, Laos. Thiland has refused to have any part of a demilitarized zone on its territory on grounds that it is not a combatant in Cambodia.

Vietnam's new offer of a limited withdrawal became known today as Thach, in an interview, linked full withdrawal of Hanoi's forces from Cambodia to the end of an alleged Chinese military threat.

"Once the threat from China ceases we will withdraw our forces from Kampuchea [Cambodia] as we did twice before" after the French Indochina war and the American Indochina war, Thach declared. China openly backs the anti-Vietnamese guerrilla forces whose military chief is Pol Pot. Vietnam maintains that it invaded Cambodia in December 1978, largely to counter a Chinese threat to its security.

Vietnamese installed a Cambodian regime headed by Heng Samrin in Phnom Penh following a military victory in most of Cambodia, and has campaigned for its international recognition. But the United Nations, at the behest of China, Thailand and other Asian nations, has continued to see the "Democratic Kampuchea" regime of Pol Pot as Cambodia's representative here.

The United States voted to seat the Pol Pot regime a year ago when the issue went to a 71-to-35 vote in the General Assembly.

Secretary of State Edmund S. Muskie announced earlier this month that the United States will support Pol Pot's "Democratic Kampuchea" in the U.N. again this year despite the fact that "we abhor and condemn the regime's human rights record and would never support its return to power in Phnom Penh."

The U.N. credentials committee last Monday approved "Democratic Kampuchea's claim on the U.N. seat for another year. Surprisingly the Soviet Union, Vietnams ally, did not ask for a vote -- possibly because the Russians reportedly were able to count on only one other vote, that of Angola, in the committee.

Despite speculation to the contrary Thach said Vietnam will insist on a full scale vote in the General Assembly, even if his cause is defeated. He said a formal vote would "show to everybody that the circles who proclaim they are the champions of human rights are still supporting Pol Pot."

As long as Pol Pot's representatives are seated at the U.N., there is no prospect of U.N. observers being stationed at the Cambodian-Thai border, Thach said, because "we could not consider them as neutral."

In the interview, Thach also said:

The Heng Samrin government will sponsor a "very important" national election in Cambodia, probably in February or March, which will be "a symbol of the revival of the nation." The vote evidently is intended to give legitimacy to the Vietnamese-backed regime.

Vietnam "hopes for the best but prepares for the worst" from the United States after next Jan. 20, regardless who is elected U.S. president.

There is no expectation of a new Vietnamese refugee exodus to rival the earlier tide of "boat people" and other refugees. But Thach complained that Vietnamese who depart under an official "orderly departure" program are scrutinized more carefully and treated less favorably abroad than those who leave illegally.

Vietnam has suffered a new economic setback due to two typhoons that affected the rice harvest in areas which total one-fifth of the country's agricultural land. As a result Vietnamese will have to "tighten their belts" again this year and the country have to import about 3 million tons of grain from outside. Without the typhoons, Vietnam's imported rice requirement would have been only about 1 million tons, Thach said.