UNITED NATIONS, OCT. 14 -- The ninth annual General Assembly debate on Cambodia ended tonight in the usual fashion, with an overwhelming vote calling for the withdrawal of Vietnamese troops there -- but also with word of a diplomatic breakthrough.

A U.N. official confirmed today that Prime Minister Hun Sen of the Hanoi-backed government in Phnom Penh has accepted an invitation from Prince Norodom Sihanouk, who heads an exile coalition government, to meet near Paris this year.

That session, diplomats said, is likely to be the prelude to a broader dialogue in Jakarta in January that could include the Phnom Penh government and the three factions in the insurgent coalition -- Sihanouk, the Beijing-backed Khmer Rouge and a rightist group led by Son Sann.

The Jakarta gathering, in turn, could include representatives of Vietnam and Indonesia as the interlocutor from the Association of South-East Asian Nations (ASEAN).

The initial dialogue between Sihanouk and Hun Sen would be a first. The idea for the four-sided meeting in Jakarta emerged in July from a meeting between Vietnamese and Indonesians. Both sides proposed a "cocktail party," because of its intended informality, but they quickly bogged down in "differences of nuance and interpretation," one ASEAN diplomat said.

Sihanouk, meanwhile, announced a leave of absence as head of the coalition, which holds Cambodia's U.N. seat but little of Cambodia. The sabbatical, diplomats explained, enabled Sihanouk to act on his own.

What Sihanouk proposed was a three-day "picnic" at his suburban French villa between Nov. 15 and Dec. 30. He set two conditions -- that Hun Sen approach him as a loyal son of the "father of his country" and that the discussions be on the public record -- possibly even recorded by television cameras. (Sihanouk also cautioned that only two meals a day should be served because he was on a strict diet.)

U.N. officials confirmed today that the positive response from Hun Sen was delivered to Sihanouk in New York on Oct. 5 by Natwar Singh, minister of state for foreign affairs of India, which maintains relations with the Phnom Penh government.

During the two-day assembly debate on Cambodia, diplomats from all sides signaled their blessings for the dialogue.

The vote on the annual resolution calling for the withdrawal of Vietnamese troops from Cambodia was about the same as last year's -- 117 to 21 with 16 abstaining. In 1986, the total was 115 to 21 with 13 abstentions.