The most important thing is to make space for the vocal. If your track is already multi-band compressed, and has lots going on in it, there probably isn't room for a vocal in there. Generally, if a track is interesting enough to work as an instrumental, it's got too much going on to have a vocal as well. Take tracks out that overlap with the frequency range that the vocal sits in.
Secondly, pay attention to the stereo image. In an instrumental, you'd normally have the lead sound panned dead centre. In a vocal track, that's where the vocal has to go, so you need to move the lead line left or right a bit to get it out of the way. Try layering a slightly different sound and panning one left, the other right, to create a gap in the middle for the vocal.
Lastly, and most difficult, is reverb. The acapella probably has a subtle very, very expensive high-end reverb applied to it already. Shoving it, and bits of your track, through a cheap low-end reverb won't make things gel properly, you need to be sympathetic to what's already been done to the vocal, and get as close to the same effect on your track as you can.