Vienna was essentially a stop over destination for us on the way through to Prague. It is a lovely city with rich architecture and people. Unfortunately for us, the night we caught the overnight train to Vienna it was daylight savings. This meant that the train had to stop in the middle of nowhere for an hour, so not to affect the arrival time into Vienna. We also arrived on a Public Holiday which meant that a lot of shops were closed. This wasn’t too much of a problem, as we took advantage of the lack of people on the streets to do walk around the Vienna Ring Road (Vienna Ringstrasse).
The things I would recommend in Vienna would be to visit Stephanplatz (their main shopping district) and try their bread dumplings, and their beer. Not a big beer fan, but I found one that I really liked there.