THE Beach Boys are alive and well, and have pencilled a gig at China Beach in Vietnam into their diary for 1994, a Hongkong entrepreneur has told us. Bruce Aitken, who was the driving force behind the last two Vietnam marathons, is now signing up bands for a surfing party in Vietnam to beat all Asian surfing parties. He is determined that the country's first professional surfing competition, taking place in October at the beach where American GIs used to go to ''catch some tubes'' during the war, will become an annual event. ''We've got some old California beach bands to play this year,'' he said. ''But next year we're pushing for the Beach Boys.'' According to Aitken, it takes a lot of planning to give a 1993 international competition the laid-back feel of a 1970s surfing festival. ''The main difference is that we also have to be commercial.''