The short answer is you probably can't do that. There are ways to remove vocals (and everything else panned to the center), but it doesn't work well with MP3 files. However, if you have a receiver that supports surround sound, you can try its Dolby ProLogic II decoder. That take stereo music and routes the center content to one line output, and the left and right channels to those outputs which you'd then record as a Wave file. Again, it may or may not work, but it's worth a try if you or a friend have a surround receiver.
--Ethan