There are plenty of reasons why you should pop into a Viennese supermarket on your visit.
First, as you might expect, they're very cheap compared to the prices you'll pay in souvenir shops, tourist outlets etc. So if you want to buy drinks and snacks, they're considerably better value. Every supermarket runs regular special offers where you can commonly get 25-50% off selected items. Snacks are often discounted like this.
Second, supermarkets sell many of the typical souvenirs of a visit to Austria. Drink, sweets/candy and chocolate, for example. They all stock Milka chocolate and the famous Mozartkugel (marzipan/chocolate balls), in the same packaging as elsewhere, and at a fraction of, for example, the airport price.
A Billa in Vienna's 17th district
In fact, if you leave your shopping until the airport, look out for the supermarket trolley signs in the check-in area that lead you to an underground Billa supermarket. The prices there are the same as in the city, and you can buy drinks, snacks and chocolate for much less than in the airport shops.
You may be surprised to find so much organic food sold in conventional supermarkets. Austrians are very food-oriented and environmentally-aware. So organic food is part and parcel of standard supermarket fare, especially dairy products.
As for picking the right chain, well, there are no bad supermarket chains in Vienna. But here's an overview of the common ones...
Merkur - my favorite. Always large, clean and well-stocked with top quality food. There aren't too many around though.
Billa - varies in quality depending on the location; some of the older stores could do with a refit. Probably the most common one you'll find. Usually good value. Merkur and Billa are owned by the same company.
Spar - comes in three flavors. "Interspar" is the large version, "Eurospar" the next version down and "Spar" is the smallest. You'll also find "Gourmet Spar", which is more upmarket in its selection and presentation.
Hofer - owned by Germany's Aldi and similar in style. Extremely cheap, but not as well stocked as the above and you won't recognize many of the food brands.