Usually in FL Studio, I make a clicktrack with the closed hihat. One hard hit one softer... ya know it...

Then I put the accapella in one audio channel and turn the BPM up/down until it fits to the clicktrack.

If it is a natural accapella, sang to a band playing, you will experience more or less, that the vocals go off the riddim, because your riddim has 100% pure digital accuracy ;-) . Cutting and slicing the vocals (accapella) is the only option for this. Then you realign the vocal slices to the clicktrack.

Usually I cut all silent part of an accapella out, keep the parts where something is earable and then align those parts to the riddim.

Hope this helps. If not, I'm here ;-)