The only easy and successful ways of removing vocals are:

• Find a stereo track where the vocal is on one channel ( say, left-hand ), and backing is on the other. Delete the vocal channel. Pan the remaining channel center so you get a good mono backing track.
• Find a stereo track where the vocal is panned exactly center. Invert one channel and mix it with the other. This will cancel out the vocal, leaving a somewhat depleted backing track. There are various programs which do this, but the result is not good.
• Get hold of the original Master and re-record it with the vocal muted. You'll need friends in high places for this one but it's the most successful way, which is why decent karaoke discs are expensive.