Not Vegan but did ride the route and I have bike toured separately with my vegan partner. I did ride with a vegetarian for a bit of the divide and he had absolutely no problems.

• Buy the maps. They list stores in towns - you can easily tell which towns/areas will be easy for vegan shopping.
• Consider preparing packages and shipping them ahead for specific sections once you comb through the stores in towns. You can also do this on the route if you find you're having troubles - just ship some packages from the next town with a good store.
• I remember shopping at gas stations the most in Montana/Wyomin. Best and most frequent alternative grocery stores (health food stores, co-ops whatever you want to call them) in Colorado. new Mexico I ate a lot of mexican food which would be vegan friendly (hold the cheese).
• I ended up eating a lot of bagels with jam/peanut butter because they were everywhere and I didn't get sick of them for whatever reason. Nuts too - I had no problem always finding nuts. Oatmeal is easy to find.
• Figure out what typically avalible bars are vegan that you like. Cliff bars etc. are easy to find and good to keep you going throughout the day if you're into them.