The main problem in creating a cappelas is generally the percussion, and any instruments that have a similiar pitch to the vocals (I don't know if pitch is the right word, I know nothing about music except how to make it). You can quite easily turn the percussion down to a warbly murmur though, and then bury that. I might be speaking out my arse here, but one of the things I've started to do, which really helps with the sampling, is to whack a very low, almost subsonic white noise oscillator under the entire track, like quiet, even record hiss. It works in kind of the same way as a wild track for doing film audio; stops samples jumping out of silence at you and buries background noises. Another thing that works sometimes if you're just interested in the vocals is whacking on a certain (fairly small) amount of reverb, which won't affect vocals as much as it will other sounds. I mostly work with non-music samples and drum-loops though, and as I have said, my theoretical knowledge is laughably small. I just press the pretty sound buttons.
