Thailand and Vietnam, in a new phase of diplomatic jockeying over the future of Cambodia, held their first face-to-face talks in months on that subject last week at the United Nations. The exchange ended in sharp disagreement, but the mere fact that talks were held was regarded as significant.

Thai Foreign Minister Siddhi Savetisila who was in Washington yesterday for meetings with Carter administration officials, said his two-hour conference last Wednesday with Vietnamese Foreign Minister Nguyen Co Thach produced no movement toward agreement on a way to obtain the withdrawal of the 200,000 Vietnamese troops in Cambodia. However, Siddhi reported that he and Thach agreed in principle to meet again, though the time and place are not agreed.

Last week's meeting was sponsored and arranged by U.N. Secretary General Kurt Waldheim, who has been trying to generate a dialogue between Vietnam and its Asian neighbors as a means of defusing and eventually resolving the Cambodian problem. Waldheim attended the first few minutes of the meeting, which was held in a U.N. conference room, before leaving the discussion to the two foreign ministers.

According to Siddhi, the Vietnamese official repeated Hanoi's previous position that a demilitarized zone should be created at the Thailand-Cambodian border to prevent the conflict from spilling over into Thai territory and facilitate stability in Cambodia that can bring about eventual Vietnamese withdrawal. As in several recent conferences, Thach suggested that Vietnam might withdraw a limited number of its troops from Cambodia if "peace, security and stability" are established at the Thai-Cambodian border.

Thach previously has refused to say how many troops might be withdrawn under such circumstances, and he did not specify a number in last week's talks. Siddhi said he had learned from a senior U.N. official that Thach spoke of the possible withdrawal of 10,000 Vietnamese troops in a talk with Waldheim . Vietnamese sources would not confirm yesterday that such a figure had been cited.

Whatever the figure, it is of more rhetorical than actual significance at this point. Siddhi reiterated Thailand's refusal to accept the Vietnamese-sponsored demilitarized zone, part of which would be in Thai territory. And U.S. officials said a token Vietnamese withdrawal would be meaningless without a system of verification to make sure it is carried out.

On behalf of Thailand and its allies in the Association of Southeast Asian Nations (ASEAN), Siddhi presented Thach with a proposal for an international conference to resolve the Cambodia problem through Vietnamese withdrawal "within a specified time-frame" under U.N. supervision. The plan also calls for U.N.-supervised elections in Cambodia and "guarantees that an independent and sovereign Kampuchea [Cambodia] will not be a threat to any of its neighbors."

Siddhi said the People's Republic of China, which Vietnam considers its main enemy, has indicated through diplomatic channels that it would be willing to provide such guarantees to Vietnam as part of a Cambodian settlement. China has also aroused diplomatic interest by taking the position in recent talks -- including those at the United Nations between Secretary of State Edmund S. Muskie and Foreign Minister Huang Hua -- that it will be willing to discuss the Cambodia problem with Vietnam before the withdrawal of Hanoi's troops from that country.

While the jockeying for diplomatic position continues, large numbers of Vietnamese troops in the western part of Cambodia continue to create tension in Thailand. Siddhi said his intelligence is that two-thirds of the Vietnamese force in Cambodia is now in western Cambodia.

"The Vietnamese want to scare Thailand," said Siddhi, "but they don't scare us a bit."