It's really hard to do that and do it well on Acid. You're better off going with Virtual DJ. I know in the older versions of Acid (4.0 and 6.0) that you could play around with the EQs and use a bandpass filter. I don't know why they took that out in 7. But a bandpass filter would allow you to keep frequencies between a certain range and filter out the rest. It was pretty dope...so dope a buddy mine refuses to update his Acid 4.0 lol.
And Abhi/Karan are totally correct. Basically, you line up the song and the instrumental. Once they are perfectly aligned or as best as you can, you essentially subtract (which is what Karan is talking about by inverting the track) the instrumental from the song. The common parts of the sound waves cancel out leaving whatever isn't common - the vocals.
Danny, I totally feel where you're coming from. To remove or extract vocals you can use Sound Forge I think. If you want to remove the vocals and if the vocals are panned dead center in a mix, you can subtract the left channel from the right channel thus removing the center channel but it will remove everything from the center channel, not just the vocals. This is best done with Sound Forge Channel Converter which already has a Vocal Cut preset.
So to summarize, try Sony Sound Forge (I think I'll check that out now too haha)
