The first one you're talking about is Carly Simon's "You're So Vain", but even she hasn't told who it is about. Rumors have been told that is was about everyone from James Taylor, to Mick Jagger to Warren Beatty.
As for the second one, A group singing about a girl... You're gonna have to get more specific
My guess is KISS singing "Plaster Caster", even though it does not appear on any of their greatest hits packages.