Depending on the method used to remove the vocals, it might be simply modified to keep the vocals and remove everything else.
Barring that, if you have the non-vocal track, you can load the original track into your favorite sound editor and subtract the instruments using the non-vocal track.
Just fair warning, the results are probably not going to be that pretty.
You could try the Voice Trap plug-in here:
http://www.cloneensemble.com/vt_main.htm. It's shareware, so to get the full functionality you have to pay $25, but you might get decent results with the default/limited settings.
Or the Extra Boy
http://www.kvraudio.com/get/1651.html and Extra Boy Pro
http://www.kvraudio.com/get/2107.html VST plug-ins.