The city itself isn't overwhelming, but it is just quaint enough to give you a personal feel while also offering everything that a major city can offer. I highly recommend a minimum of 3 days to immerse yourself in everything, and more than a lot of other cities, Vienna is one that outside a planned trip to a heurigan, is a perfect place to not schedule any visits, but just walk around and experience it intimately.
Insider Tip:Heurigans are wineries at the outskirts of the city, and are by far the most amazing thing I have ever experienced abroad. Granted we had a perfect day for it, but me and a few friends went up to the heurigan, and drank their freshly brewed wine and talked for 8 hours without ever noticing the time going by. The heurigan we went to, Weinbau Zawodsky was perched in the countryside on a hill, and looked out over the city of Vienna. Keep it quiet though, we were the only non-Viennese, and that made it great.