One of the best tips I read about Vienna was visiting the sky bar for a drink at about sunset. Yes, the drinks are a lot more expensive for a rooftop bar, but you are paying for the view and, when I went, even had a bit of jazz playing at night.
If the temperatures are not too cold, or the weather not too ghastly, try to nab one of the tables outside for a better view of the surrounding rooftop areas.
You can find the skybar on the main pedestrian shopping esplanada. Look for the glass elevator, or simply look up the address.
Name: Skybar
Found at: Kärntner Straße 19, 1010 Vienna, Austria
Website: www.skybar.at