A cake mix by itself is vegan, but often not the ingredients given in the directions. You can still use a cake mix and substitute your own vegan choices, like flaxseed for eggs and almond milk for milk, or just water. I couldn't believe it when I read how to make a vegan cake using cake mix and Coca-Cola! I don't drink Coke, but my thought was to try with orange juice and canned fruits. Coca-Cola is not strictly vegan, for it contains fish gelatine that stabilizes the color.
I've made an infographic. Check out my vegan cake mix substitutes below: