Mobil Oil Corp. has begun discussions with officials from the Soviet Union and Vietnam over a three-way deal to drill and explore for oil in the South China Sea off the coast of Vietnam, the Fairfax-based company confirmed yesterday.

Any deal depends on an end to the United States's embargo on trade with Vietnam, and the company emphasized that the State Department has been fully informed of its conversations with Vietnamese officials.

If the three-way arrangement is set up, it will be the first joint venture between U.S. and Soviet companies for operations in a third country.

The deal was reported yesterday in the Asian edition of the Wall Street Journal, which said Mobil first broached the possibility of moving back into Vietnam at a New York meeting with Vietnamese officials in September.

The development of Vietnam's oil potential took on added importance after Iraq's Aug. 2 invasion of Kuwait.

Mobil is looking particularly at three areas that it had explored in the South China Sea in the 1970s, when it purchased concessions from the government of South Vietnam.

One of a number of American oil companies that bought concessions from the Saigon government, Mobil was the only one to strike oil and to begin developmental drilling.

Then the government fell in 1975 and Mobil pulled out.

"If the United States government changes its relations with Vietnam, we are in a position to start to resume some business if we can," said Thomas Collins, public affairs manager for Mobil's exploration and production operations.

During his meeting in the fall with Vietnamese Foreign Minister Nguyen Co Thach, Secretary of State James A. Baker III said the United States is ready to normalize relations with Vietnam when Hanoi supports a settlement in Cambodia and provides assurances that there are no U.S. military personnel or their remains in the country.

Although a Vietnamese-Soviet joint venture, named Vietsovpetrol, is pumping oil at the tracts explored years earlier by Mobil, Vietnamese authorities believe they could get greater production using American technology as well as technical data developed earlier by Mobil.

Added domestic production will become more important to Vietnam next year when the Soviet Union begins charging hard currency for its oil exports.

A Vietnamese oil official was quoted yesterday in Platt's Oilgram News as saying that the United States has superior technology, especially for drilling in shallow water such as the South China Sea.