So I was annoyed by the blinking light that my desktop has when it's sleeping, and decided to put it into hibernation. I was then further annoyed by the lack of a hibernation option in the Vista shutdown menu, and even more annoyed when I couldn't find a way to re-enable hibernation. Via Google I then found several sites confirming that there is no way to enable or disable hibernation in Vista using the GUI (I think I disabled it by removing the hibernation file in Disk Cleanup). However, it is possible to enable or disable it via the command line, and you can read about it at those sites I linked to above.
Instead of doing this though, I have found it just as convenient to use the command shutdown -h. Just type that in the search box in the start menu and hit enter, and the computer will drop into hibernation, whether the menu option is there or not.