this is extremely difficult to do unless the vocal is actually on its own. If its on its own use an audio editor to cut it from the track.
If its on a music track with music playing over it i wouldn't even bother trying, its too difficult to get it without the rest of the background music, in fact its nearly impossible.
You could try finding out where the vocal comes from, say if its from a specific movie then get the 5.1 version of the movie and use ripping software to get the centre channel of the 5.1 stream. this will be the vocal stream and should be clear of any background music etc.
all the best,