Food Stores

https://www.quora.com/Where-can-we-get-the-best-street-food-in-Vienna

enter any Spar, Billa or whatever and go to the sausage counter

tell them to make a roll with “that” sausage (pick any you like – and there is a huge selection .. also a huge selection of hams and bacons)

he salesperson will take a fresh roll (crunchy and perhaps still slightly warm from the oven) and cut fresh sausage to fill it for you – if you want to ask to add a small pickle or some mustard that perfectly fine (instead of the standard roll, you can akso pick another one if the bread counter has them on stock)

the cheese counter will do the same thing with any cheese you want to have …

just ask for “Eine Semmel mit xxxx Wurst” (and if you want more then one, that is not a problem either

many supermarkets will also have some hot food like a Leberkäse or similar
https://www.google.com/search?source=hp&ei=QW2cXOj7OsfYtQXVvq3QBg&q=leberkäse&
bologna sausage