Windows Vista unlike the previous versions doesn't have an option to enable or disable hibernation in the Power Options of the Control Panel. Strangely, if you search for hibernation in Control Panel the Turn hibernation on or off option will be found but it will only open the Edit Plan Settings dialog without hibernation available if it is already disabled. Therefore, if you end up with disabled hibernation as I did (probably due to the Hibernation File Cleaner option of the Disk Cleanup tool) the only way to turn it back on is to open a command prompt and run

powercfg -H on

This will make the hibernation reappear in all the shutdown menus and power button combos.