if i got both the instrumental and vocal version of a track?

anyone wanna to it for me?

http://www.youtube.com/watch?v=pRDAA5vQ1g8

http://www.youtube.com/watch?v=esTVVjpTzIY

If the instrumental part of both is the same (same phase blah bla blah)..
then the only thing you should do is to load both files in your host and reverse the phase of the one and play the files together...and get the vocals.To do that..you must trigger the waveforms from a specific point at the same time and the gain of the instuments must be the same.

If there is no effect you're doing it wrong or the 2 instrumental parts are not the same.

If some bugs appear in specific "times" in the vocals it's probably because of the side-chain on the vocal version between vox and an instument.

If reversing the phase do not work at all well....good luck
You'll have to have to use noise removal gears (like WAVES) in extreme settings, sharp notches to sweep out harmonics of instruments and lot's of patience&luck but be ready to loose vital parts of the vocals like warmness ect

sounds like alot of work. Maybe i give it a try later. Just to much work for remixing a shit tune like this.

Yeah don't try it unless you realy want too.
If the phase reversing doesn't work it ain't easy at all to unmix a track...
it's a studio engineer work.

There's nothing you can do to get a good a capella without being magic, or having a copy of the original recording files.