This thread was very useful for me, thank you! I ended up with slightly different settings than is discussed above, though.
As you can see, I ended up having to send the vocal signal to the BUS, not to the plugin itself. The midi does go to the plugin, though. The final I had to get over was remembering to turn on the Input Echo in both tracks when I made my initial recording
I also wish the TAL vocoder plugin had visual feedback indicating it was, or wasn't getting signal, to help figure out what I was getting wrong along the way, which was everything.
This setup let me do several things I wanted:
1) have my own synth sound in Vital as the carrier
2) Be able to play and sing live to myself, to work out what I wanted to do, appromixately.
3) Record both singing and playing keys each to their own track
4) Play it back and also be able to edit the midi to stack up more harmonies than I can play live.
The remaining bit I don't fully get is the panning. Trying it just now I was able to put the Voice panning anywhere I wanted, both the track pan AND the Send pan, and my singing robot self was still there.
Also the MIDI track pan could be anywhere, but the Midi Send pan had to be center OR right. I know the TAL plugin is listening on the left and right side of the stereo signal for its input, but would that be the track pan or the send pan settings here?
I'm happy today, though, I finally got this vocoder thing working, thanks to all of you!