It's very easy to recognize all officials Vienna taxis because the cars are usually marked with a sign on the roof. This sign with the inscription -TAXI- is illuminated during the operating hours. On the city-walks map you can find some of the Vienna taxi stands in the center of the city. These are marked on the city map with the symbol "TX". The cheapest and quickest way is to go to one of the stands. If there is no car available then you should use the phone at the stand to call a car because for that you have to pay the lowest extra charge for the taxi fare.
On Sundays, public holidays and in the night between 23:00 and 6:00 o'clock you have to pay approx. 15% more for the fare. One ride in the center of Vienna normally doesn't cost more than 15 Euro. For a ride to the outer districts you have to calculate with up to approx. 25 Euro for the fare. The costs for a Vienna taxi ride, can of course vary depending on the actual traffic situation.
Links: Online Fare Calculation