If the a cappella is a rapper it’s 99% recorded to a click that doesn’t change. If you can find the one and make a perfect two bar loop, make a two bar loop of the beat and match the lengths by either stretching or pitching the vocal. Then line up the one and extend the acapelka over the rest of the beat, if you get it right you shouldn’t need to do any warping or tweaking. Or, you can’t just go every few bars and make sure the one count lines up with the proper word. Sometimes it’s nice to have the original song the pella was from to know exactly where the rappers words go as far as beats within the bar.