In May OeBB site wouldn't take credit card payments. I ended up booking on Czech Railways (which btw was a pretty good train ride).
For VIE-BUD, I again had the same issue with OeBB. Hungarian railway tickets booked online needs to be picked up in person in one their office and their pickup offices are only in Hungary. So, technically that rules Hungarian Railways as an option for online advanced booking for non ex-Hungary trians.
You can however, always walk up and buy ticket. I needed them in hand for schengen visa.