You can't magically erase vocals from a track par-se. However, you can emply a technique that often has this as a result - called centre-channel cancellation. Take your stereo file (if it's mono, you won't get anywhere), and invert the phase of one of the tracks (eg left). Now combine the 2 together, into a equal mix - so you get a combination of (-L) + (+R). The result will be the removal of content that was centrally panned / in phase. This is usually vocals, lead melodies and parts of drums. However , you will get different results with EvERY song you apply this to. Some successfull, some terrible.