Big time difference: the Vocaloids could sing something entirely different from the lead if you're talking about real-time harmonizer use. They can sing in ways humans cannot (if there's a use for that). I think they have the potential to sound more realistic than a harmonizer. They can do 16-part harmony, solo lead lines, scats, and other strange and exotic things that no harmonizer can do. The biggest thing is the full editing power of the program that allows you to tweak every single phrase, word and note to extremes. I would think of it as a synthesizer/sequencer (which it is) to get the full picture. The more I think about this program, the more possibilities I come up with.