The difference is hard to tell. I own a V256 since a couple of days now, and it’s a vocoder and harmonizer as well. The V256 has MIDI in, which the voicebox has not. The midi in allows you to play the internal synth of the V256. This internal synth cannot be isolated (so in fact, you don’t hear the synth), but it is internally used to modulate on the voice (vocoding) or for note-tracking and harmonizing.

The voicebox / V256 overlap in functionality as far as I can tell. There is a thin line between vocoding and harmonizing. Maybe the V256 is more focussed on vocoding.

I used an external synth (novation A-station) as input (carrier) for the vocoder, and then you have the ultimate control over the sound of the vocoder: The external synth AND the knobs on the V256 can be used to create the most extreme effects.

It’s a pitty that the V256 has only a mono output, so when used to harmonize, all the voices are mono. Obviously, an external chorus/reverb can be used to finish the sound. (the voicebox had a built in reverb).