If you arrive at Concourse D, it is hardly a short walk. The Arerotrain station for C is at the far end of C and is located off a "spit" where the new Concourse C will be built some time in the distant future.
The larger issue here is ontime performance. There is no way to predict this. It is a legal connection so UA will rebook you, but the question to you is the consequence of being delayed into VIE by somewhere between hours (routing through FRA/MUC on UA-LH) or by a day for the next VIE nonstop.
If there are already delays when you get to ORD, have in mind some options as UA might proactively rebook you from ORD.