Try using another driver. It seems in some cases the directX driver gives better results than Asio. Just try every driver in turn. That's what I did on a old Pentium4 running XP with a low-end terratec sound card: Asio gives 40ms, directX gives 12ms. Go figure...
good luck !
Charlot
With the directx drivers selected audio drops out till latency is set to 150ms. even midi glitches till then. I didn't think that Vista used directx drivers but evidently there is some kind of directx layer in there.