Its actually a talkbox in california love not a vocoder. Vocoder means vocal coder de-coder, basically computerised. Whilst a talkbox is basically a speaker in a box with a tube going out of it, which you put in your mouth and form words. Its REALLY hard to use though.
This is what made Roger Troutman so unbelievably talented, because for every syllable you want to say with the talkbox you have to press a note on the keyboard or synthesizer your using. Ive tried it and I can just about manage to say "baby". If any of you ever get the chance to try one you should!