I am just trying out the demo of Vocoder V before investing in the collection. ....
OHO!
After some further routing experimentation in Reaper ...
I decided to try doing the routing "backwards" from what the instructions say.
On the Vocoder track, set the input to your microphone rather than MIDI input.
Create a new track with no audio and only MIDI input.
Route the MIDI track to the Vocoder track, but ONLY the MIDI. Disable audio send.
And there ya go. Worked on this end. Hopefully that works for you as well.