"Miss Saigon," an updated version of Puccini's "Madama Butterfly" set in Vietnam as the Americans are fleeing Saigon, opens at London's Theatre Royal Drury Lane on Wednesday. It's the new show from the French team that reworked Victor Hugo into the theatrical hit "Les Miserables," librettist Alain Boublil and composer Claude-Michel Schonberg. Comforted by a hefty $7.5-million advance sale, Boublil and Schonberg now await the judgment of the critics. "Naturally, we are nervous," Schonberg confesses. "Miss Saigon" marks the first time the two have written in English. In the story, "Miss Saigon" is a teen-age prostitute who falls in love with an American G.I, has his child, ends up working in a Bangkok nightclub and commits suicide to give her son a new life in the United States. Lea Salonga, 18, a child star in the Philippines with extensive experience in musicals, plays the title role.