Alternative vocal isolation technique

Hey Folks, forgive me if I’m asking a newbie question.
I have managed to isolate vocals from a song by inverting an available karaoke track and then mixing them to cancel out everything but the vocals. This works great if an actual official karaoke track with just the vocals removed is available.
My question is this: If I have 2 or 3 different remixes of a song which all have the same (or very similar) vocal tracks in them, is there a technique for isolating the vocal track that is common to all of them? I can’t think of a way off the top of my head, but hopefully someone here is smarter than me. Thoughts?

If the vocal and only the vocal is dead centre in the mix, then the “Blue Cat Widening Gain” which is available as part of the free “Gain Suite” http://www.bluecataudio.com/Products/Product_GainSuite/ can isolate the vocal. If that is not the case then it probably can’t be done without either an identical recording of the music (without vocal) or access to the original multi-track recording.

Well, that answers that. Thanks =)