http://www.pilotguides.com/articles/vienna-street-food/

Pilot recommends:
Café Hawelka (Dorotheergasse 6) – an institution in itself, the Hawelka has been run by Mr. & Mrs. since the end of the 2nd World War. Very dark and smoky, you get a feeling of turn-of-the-century intellectual Vienna.
Kleines Café (Franziskanerplatz 3) – a lovely place to come in summer, as the seating outside is in the pretty Franziskanerplatz. The service is friendlier than many other places and you can get good food here too. The atmosphere is bohemian.
Café Alt Wien (Backerstrasse 9) – Covered in posters advertising everything from concerts to demonstrations, the Café Alt Wien feels like a good place to come if you’re plotting a revolution. It dispenses with the tuxedoed-waiter idea too.