Vienna is a very well known city. It's not the same as using the name of some small, unheard of town.
By big and well-known I just meant "famous" in the same way that Paris and London are, as I mentioned. I did an A-Level in Geography so I definitely know capital cities :) Vienna is no doubt an important city, but it's not seen in the same way that places like Paris, Milan etc are. But as amberdaydream said, it's been around as a name for a long time, I think that makes it even more acceptable.
That's my boyfriend's niece's name. :D
I'm not sure I like Vienna as a name. The fact there are two vowels right next to each other and a -na/-nna ending make it just a tad too frilly for me. Not that I'm much into geographic names, really. The fact it has some history to it though makes it not too bad. I can see why others would like it; it's just not for me.
I love Vienna as a name. It sounds so soft and flowy to the ear. It's actually one of my favorites!