Soviet oilmen searching for crude in the waters off southern Vietnam for the past two years have installed at least one drilling platform and hope to show results this year, according to western sources here.

The effort, if successful, would greatly boost the weak economy of Vietnam, which is rated among the world's poorest countries, diplomats said. But it is also being watched for clues about the state of Soviet offshore oil technology, and for its implications in Vietnam's feud with China.

Some diplomats feel the Soviets may literally be out of their depth. Up to now they have drilled for offshore oil only in the relatively shallow waters of the Caspian and Black seas. The area the Soviets are exploring now is in much deeper water off Cap St. Jacques.

"The Soviets are using the same techniques they used in the Caspian and the Black Sea," one diplomat said. "They don't have deep-sea technology."

He added that the platform the Soviets reportedly are using was purchased in Norway, which along with Britain has been producing oil in the deep and often rough waters of the North Sea.

According to diplomats and other western sources, the Soviets signed an oil agreement with the Vietnamese in 1981 after Hanoi gave up hope of bringing in major western oil companies to undertake the task. The French company ELF negotiated for six months, but pulled out when the two sides reportedly were unable to agree on a price.

The Vietnamese have told diplomats they expect the Soviet exploration to show commercial quantities of oil this year and that they hope production can begin between 1985 and 1988. It is not known how the Vietnamese arrived at these targets.

The Hanoi government hopes that production will be enough to meet domestic needs estimated at 2.2 million metric tons of oil a year, a diplomat said. Vietnam's oil requirements currently are met by the Soviets, whose aid keeps the economy afloat.

The results of the Soviet efforts may also have important implications for a Sino-Vietnamese dispute over two groups of islands in the South China Sea known as the Paracels and the Spratlys.

The dispute has intensified since China, which controls the Paracels, signed contracts with the Atlantic Richfield Co. for oil exploration between Hainan Island and the Paracels and with the French firm Total for drilling in the Gulf of Tonkin. The Chinese reportedly plan to sign further deals soon with western companies for more oil exploration off Hainan.

Last month Vietnam accused China of violating Hanoi's sovereignty by allowing the foreign oil firms to operate in the disputed waters.

A statement issued by the Vietnam News Agency said that, based on "firm historical and legal data," the Hanoi government "has many times affirmed its sovereignty over Vietnam's territorial waters and continental shelf as well as over all the natural resources in those areas."

"The foreign companies that conduct the exploration and extraction of oil and gas in violation of Vietnam's sovereignty must bear all the consequences arising from their wrong actions," the statement warned.

The Chinese reportedly have told the western oil companies to disregard the Vietnamese warnings and have threatened military retaliation against any interference in the firms' operations.

The Vietnamese currently are producing small amounts of natural gas in the Hanoi delta sufficient to supply some northern provinces, and diplomats believe authorities eventually would like to explore for oil offshore in the Gulf of Tonkin.

Thus, the sources said, if the Soviets are successful off southern Vietnam, they might be put in the awkward position of being asked to drill in the disputed waters themselves.