There's room in South London for more than raunch. This 18th-century brick relic was once a music hall and drag cabaret, saved from demolition by local gay businessmen. Saturdays at the Royal Vauxhall Tavern, screamers of all genders turn out for Duckie, a busy dance floor playing indie and techno trash till 2 A.M. The revived pub-theater also drags drag, burlesque, and improv into the present with nightly shows-among them, Sunday's popular Dame Edna homage.