Kinda hard for me to answer when you don't say which vocoder you are using.
But, some of them have a MIDINOTE module which will receive MIDI note messages (e.g. 6812) and others do not (e.g. 6810).
It's probably a matter of picking the right preset, or, if you have Vsig skills, adding MIDInote functionality to one that does not have it.
