One of the oddest exhibitions of "infotainment" ever broadcast is due to air tonight as a two-hour ABC-TV special, Elvis: Viva Las Vegas, emceed by 20/20 co-host and former ABC World News anchor Elizabeth Vargas. (The show received a lengthy promo last Friday, delivered by Vargas's colleague, John Stossel.) The special includes never-before-broadcast concert performances by Presley, performances of Presley tunes by such singers as Paul McCartney, Chris Isaak, Brandi Carlile, Joe Perry, and Toby Keith. It also includes interviews with other pop stars about how Presley may have influenced their careers.