I'm surprised they only require B2. If I was going to do a PhD, with classes in German, I'd want to have at
least C1 level. And not just manage to pass the C1 test, but
really be C1.
Still, I have heard of universities that only require B2 level to get in. But a PhD in Vienna? That'd be hard in my opinion.
Still, never hurts to get the B2 and see where you go from there.