For a real time audio sample of the Digitech VR in action, check out: www.boydsongs.com
and go to the Nightingale song.
Much of the realism in any harmonizer comes from balance and understanding. You need to think like a vocal ensemble, and not just a soloist with some digital help. There's a whole school of thought that aids the creative process when dealing with vocal harmonies. Listen to Manhatten Transfer or Four Freshman or Beach Boys or Take 6 for more of what I mean.

Singing harmony is an art all unto itself. Do the study, and then do the performance!