I’ve used TAL Vocoder a couple of times, and this seems to be the easiest way to do it – I put melody on one track, vocal sample on another, and the vocoder plugin on a third, then use track sends:
- Carrier (i.e. melody line): Pan hard left in instrument settings, send to Vocoder channel.
- Modulation (i.e. vocals): Pan hard right in instrument settings, send to Vocoder channel.
- Vocoder: make sure the INPUT MODE ON button is on.