I once did this for my guy...it worked OK but still wasn't perfect. This is what happened: My friend makes a beat. Somehow this beat gets leaked and ends up with somebody in the hood rapping over it (along with a hook sung by a girl). When we discovered this, my friend wanted me to take the hook from their song and put it into his & make a diss track. So what he wanted me to do was just see if I could get their hook by itself & place it at any location we wanted in his song. The end result was actually better than I thought...I'll post up the original (stolen-beat) song and the "acapella" hook produced if I can find it.
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The "produced" acapella from subtracting beat from vocals + beat.
http://www.sendspace.com/file/qpyve6