And now for the bad news ... it is not typically possible to isolate vocals.
If you have a vocal with no competing frequencies, such as in a breakdown where there's only a baseline (for example) then you could possible filter out the bassline ... but, in most situations there will be snares, guitars, and all sort of other sounds that inhabit the same frequency range as the vocals. No way to filter that out, I'm afraid.
Some people will tell you it's possible, but they are wrong ... :( ... though I wish you could ... there would be no end to the remixes, if that were the case.
There's a bit more about this, in this thread: