Japanese companies have set their sights on doing business with Vietnam, which many avoided for years in order not to upset the United States.

"It's Vietnam fever," said Tatsuo Torigoe, an Asia division manager of Kanematsu Corp., Japan's major trading house. "The time has finally come for us to make a move."Analysts in Tokyo believe Japanese televisions, refrigerators and other household goods will soon fill shops in Vietnam as they do in other nations around the globe.

They also expect Japanese businessmen to buy more Vietnamese petroleum. Oil already accounts for more than half of Japan's imports from the Southeast Asian nation.

Five times as many Japanese companies sent representatives to Vietnam in 1988 as in 1987 "and it is my impression that there will be more this year," said Akitake Yamamoto, a spokesman for a Kanematsu subsidiary that has traded there for more than 20 years.

Until recent months, Japanese companies were either restricting business with Vietnam or dealing discreetly.

Activity was dampened by their government's support of the U.S. policy of isolating Vietnam and by a 1987 U.S. Senate resolution urging Japan to prevent private companies from trading with Vietnam.

Although Washington still refuses to establish formal diplomatic relations with Vietnam, there are indications it may be taking a less hostile attitude..

Victor Co. of Japan, which already exports television sets to Vietnam, and Sanyo Electric Trade Co., a purveyor of home appliances, both plan to open offices in Ho Chi Minh City.