I`m dealing with the voice more and more these days, and no, there really isn`t any decent vst for vocal twisting purposes.

I would suggest however the following effects for vocal manipulation.

Ring modulation.
Pitch Shift/Harmoniser
Chorus and flange with high lfo modulation
Distortion
Convolution effects.
Vocoding.
Buffer overide.
Oh my god (ohm force)

There are specific plug ins I like to use, I can pm you if you are interested in that.