Here some notes about fast food traditions in Vienna.
The first McDonalds in Vienna opened in 1977. Before that fast food did not exist in the big variety as it now does. Additionally, approximately until the late 1960s, eating outside on the streets or in parks was considered as inappropriate behavior.
The oldest, most likely the very first, now traditional, fast food in Vienna was invented more than 120 years ago and is served ever since more or less unchanged (and eaten inside). If you see a place called Trześniewski, you should pop into and give it a try.
A Polish immigrant bearing that unpronounceable name created these little rectangular pieces of brown bread covered with 30+ different spreads. I love it, although no all flavors.
Here you can see the pictures: https://www.trzesniewski.at/shop/broetchen
Usually people eat 3 or 4 pieces and drink a Pfiff Bier (a "whistle of beer", 125 ml or 4 fl.oz.).
Although this the oldest fast food in Vienna, it is not the most popular, because it is considered as "upper-class" fast food. Ordinary people, in former times blue-collar workers, but now practically everybody, go for different variants of sausages offered at snack stands everywhere in the city.
Their offerings are closely watched by the locals, discussed in newspapers, and ranked and rated in lists. To get an overview watch this short video (having English subtitles and commentary), in which a renowned chef of a famous restaurant is glossing this topic: https://www.youtube.com/watch?v=2jMdLfZ7sDk