I think Vista are quite expensive for bulk prints; the 100,000 for c£700 is pretty good value, but if you need full colour....??
However, I've used a company called Saxoprint.com
You need to submit your artwork, which was a bit fiddly at first, but the quality and price is good. 10K full colour 350gsm, at £165 delivered.
I bought my images from an image library for around $15. Job done.
Use thinner paper and the cost is even cheaper!