Something startling happens during the early part of 'The Rat Pack Live in Las Vegas,' the West End hit now dipping its touring toe in U.S. waters. One is struck by the physical resemblance, the vocal technique and the sheer mastering of mimicry of the performers who play Frank Sinatra, Dean Martin and Sammy Davis Jr... [the] concert crowd could make the B.O. go ring-a-ding-ding.