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.
Found at: Kärntner Straße 19, 1010 Vienna, Austria