I use a Vocalist - it will not provide pristine studio vocalists, but it can be a good tool.
The harmonies do not sound completely natural - but mixed low can fatten up a sound.
I don't remeber if the Perfomer has MIDI (I assume it does) - if so, and if you use MIDI - the best application would be to control the Vocalist via MIDI.
I play the harmony notes into a sequence and then assign the sequence to the Vocalist via MIDI. When I send a vocal signal into the Vocalist, the sequence controls the notes the Vocalist generates.
What I normally do is track the vocalist harmonies and then track human harmonies - useing the Vocalist harmonies as a guide (I find I can hit the harmony lines better if I have notes to follow.
Then in the mix I blend the Vocalist with the real voices (the Vocalist is mixed lower) which gives me very full vocal tracks - like doubling everything).
Keep in mind the higher the harmony the more unhuman the Vocalist becomes (can you say Chipmonks??)