FWIW, you do not need to disable UAC, although it shouldn't matter if you do.
I saw the same problem when I tried it just now, but when I set PokerStars to Run As Administrator as well as PT3 it is working fine. Sometimes software updates can make it forget settings like Run As Administrator so please check, although if you are always running with right-click > Run As Administrator that shouldn't apply.
I have Windows 7, not Vista, but I wouldn't expect there to be a significant difference for this kind of thing.
When you start PT3 and PokerStars with Run As Administrator, with UAC enabled, it should ask you to confirm twice (once each) - are you seeing that?
