Sometimes you can find a good enough MIDI of the song depending on what you're looking for, and if the vocal is assigned to an instrument, mute that channel.
There's a trick with stereo wave files where you invert the phase and recombine, and it depends on the vocals being placed dead-center in the stereo field and the instruments all panned a bit (which is often the case). Sorry, don't recall the specifics at the moment. Invert the left channel and combine with the right and vice-versa, or something like that.