I don’t have Vista yet, but I’ve heard complaints of Hibernate not existing/working in Vista.

Hibernate not available in Vista or stopped working – from the article:

… The disk cleanup utility wiped out the file Vista uses to store a duplicate of the contents of memory when hibernating. Apparently, if that file is not present, the power features of Windows assumes hibernation is not available. Fortunately, the remedy is simple. On Windows Vista, go to Start | All Programs | Accessories and right click on the Command Prompt icon. From the menu, choose Run As Administrator. Then, click OK on the UAC prompt so the command prompt will appear with full Admin rights. Keep in mind that we assume you are logged in with an account with limited admin rights to begin with, you just need to be elevated once UAC verifies what’s going on. If you are logged in as a regular user, log out and back in as a user that is a member of the Administrator’s Group or use the built-in Administrator account. Next, use the powercfg command to re-enable hibernation by typing the following command:

powercfg /hibernate on

Go back to the power settings in Vista and you’ll notice that hibernate is now an option (if it still isn’t, try a quick reboot).

This time, when I closed the lid of my laptop, it hibernated.