Here is a simple solution on how to prevent Windows Vista from waking up on its own from Hibernate mode. The situation described is for when your computer enters Hibernation mode correctly, but somehow it resumes from Hibernation mode without you clicking the mouse, pressing any keys on the keyboard, or pressing any power/restart buttons on your system.

There may be more complicated reasons why your Vista system wakes up on its own from Hibernation mode, but this solution should be tried first to rule out the most basic problems.