I have one song that I have done this successfully with in Acid 4 (Most of my stuff is instrumental). I used the beat mapper to try to match the tempo of the acapella to my rhythm track. I then split the vocal track between each vocal pause (as many as I could get away with - some places you can't split and still have it sound natural) & nudged each phrase on the correct alignment with the beat (you need to magnify the files immensely to do this with accuracy).
This was a tedious process of course, but the results are undetectable, as long as you are not adding too much pause between splits. Also, it is not necessary to put each phrase exactly on the beat (vocalists aren't drum machines), so you can compromise a little balancing the length of pause with how much the following phrase is ahead of or behing the beat.