I hadn't ran it on vista x64, but when I had WinXP x64 installed it ran great. Never experienced any issues with it. It had some graphical improvements(like the double dmg pickup was black-purplish, and a bunch of other cool crazy things), but other than that, nothing different. Only thing different from normal x86 execution is simply running the x64 UT2004 executable.
Might be a vista thing, but I wouldn't see why. It runs the same on Vista x86 as XP x86. *shrug*