It's been a goal of mine for some time to get my AI off the ground... Having a new little one I didn't solder and bolt together with my own hands and who doesn't have an off switch has gotten in the way...
Here's the info on creating Festival voices:
To be honest, most of it is EXTREMELY Academic as it was produced for work at the University level to better understand speech synthesis...
Nonetheless, if you persevere and spend a lot of time looking up terms, you'll probably win out.
Regarding using Vocoder, I could find little to no information surrounding actually using it in conjunction with Festival or Flite.
Best of luck!