If you have problems with time sync in guest Vista please answer my questions:
1. What edition of Vista (Business, Ultimate) do you have? Build number?
Vista enterprise. How do I find the build number? This is the final released version, not a pre-release.
2. Are Parallels Tools installed correctly? Can you use, say, Coherence?
Yes, apparently so. I can use Coherence.
3. In guest OS open Parallels Tools Center -> Time Syncronization settings and check that it has status "Enabled"
Yes, it is enabled.
4. Is "Keep difference between primary and guest OS" enabled?
No
5. What is "Synchronization interval, sec" value?
60