I'm thinking this might be easiest to do with Texture Painting: Face Selection Masking. It won't be vertex colours, but I was going to bake the vertex colours anyway, giving me an image too. It would be nice to solve this thread too anyway. DPC
Regarding the vertex colour topic, I hope I understood correctly what you are trying to do. :) To achieve such hard transition between the colours, as shown in your example, you would need a lot of geometry because the colours blend between the vertices. I would rather create a black and white mask with vertex colours and…