So, this solution does expose one to some really cool things: Raspberry Pi (University of Cambridge’s Computer Laboratory); Debian Linux configuration; hardware-level general purpose input/output (GPIO); TTL serial communications; logic level converters; the Festival (offline manual in PDF format) text-to-speech synthesis system (University of Edinburgh's Centre for Speech Technology Research and Carnegie Mellon University) which has a Scheme-based (SIOD) command interpreter for control; basic soldering techniques and more!
This project can easily be completed in a weekend, and done together with a child or friend. Only a Raspberry Pi, simple components and basic soldering are required. What you will have in the end is a unit that operates very much like the high-end Votrax systems of the day. ... You OPEN a command channel for writing and PRINT the sentences and words you want spoken. Now you're talking!
I have additional VIC-20 material over on my web site: http://www.geocities.ws/cbm