Go to a coffeehouse for a truly Viennese experience. Order coffee and a pastry and sit as long as you like. You won't get a full meal, but you'll get a fascinating cultural experience. Check the prices before you order, of course, since the price of coffee can vary widely. Visit the area around the university for a number of good budget choices.
Some restaurants serve traditional Austrian cuisine, and others offer a variety of ethnic foods.
Walk a few blocks away from the main tourist areas, where food tends to be very expensive. Avoid the fast-food chains that are strategically placed to attract homesick tourists. The prices are higher than those at home, and the food will be unremarkable. Ask residents for their budget dining suggestions - almost everyone is sure to have a few favorite places. German - most people know at least a few words of English.