As the only vegan place in Mainz, Haus Mainusch is worth seeking out. It's run by the university's antifa collective, and is next to the main cafeteria on campus. If you aren't familiar with left-wing collectives, don't be put off by it. The people are friendly, the food cheap and plentiful.
Don't forget to wash your plate after you eat!Pros: Cheap, good food., Friendly people., Good politics.