Small (vegan) Gem in
Why locals love it
Be prepared to meet lots of local (music/art) students as it's located near most of their facilities. Also some local business people drop by. It's a rather small but cozy place that offers great soups, stews and cakes - vegan available and you can always bug them with any allergy questions.
Why you should visit it
They serve free water and very nice, hot meals. Not too cheap but well within Vienna prices. The idea is not new but very well executed here. Friendly people and nice atmosphere. For example try the African potato/bean stew.
Go before/after your visit to Belvedere, it's basically across the street. Take a look at Salmbräu while you're at it if you have a craving for beer.