The only thing I can think of is if the vocal is equal volume left and right, and all the intruments are more to the left or the right, you could use some system to only allow through sounds which are equal L and R, and reject all else. It would be easy if you wanted to reject the vocal, you'd simply subtract L from R (ie add L in antiphase to R). Vocal would vanish and some other instruments would be reduced in volume. But keeping only vocal - some sophisticated software would do it