Here is some advice for your visit to an Indian restaurant in Vienna:
If you are used to Indian food in other countries, e.g. the UK, you will be very likely disappointed by most Indian restaurants in Vienna. With just a few exceptions, most restaurants have adapted to the local taste, which is anything but spicy. You may even come across curries that look and taste like goulash. At any rate, curries are often underspiced, and it is extremely difficult to find a truly hot curry.
The prices include the service charge; still, a 5-10% tip is nevertheless appreciated.
Many restaurants charge between €0.75 and €2 for the cover. Check the menu in advance (the German word to look for is "Gedeck") to find out if and how much they charge; It is pointless to complain afterwards. You should receive a spotless table cloth, cloth napkins, a starter or a digestif. Still, in some restaurants you will receive neither of these.
Many restaurants offer set lunches or buffet lunches at lunchtime. My experience is that these are kind of okayish, but they are usually pared down and not really indicative of the restaurant's cuisine. You can recognize a good restaurant only if you order à la carte.