I don't know about the government regulations in Vienna, but there are cities all over that are adding new rules regarding short term rentals. Contact your local city officials to see what they are changing and how it affects you. It might not be as limiting as you think 🙂
Do you own the apartment ? Many locales only allow owners to rent additional rooms out.
Or if you also rent, then you may need permission to sublet from your landlord. Good luck !