In my quest to make great looking models using vertex colours only, and avoiding UV maps altogether, I’ve come up with a reasonable looking (IMHO) process for creating the Toon look.
The process basically involves calculating ambient occlusion levels per vertex, then combining that with the existing vertex colours. Screenshots and code after the link!
http://entitycrisis.blogspot.com/2011/02/per-vertex-ambient-occlusion.html