How to Enable Hibernate in Windows Vista
Do you realize that Windows Vista does not have the hibernate feature? Instead of hibernate, in Windows Vista, you can only put your PC or laptop in “Sleep” mode (or standby mode in Windows XP) where Windows Vista keeps your session in memory and your PC or laptop in a low power state. So that you can quickly resumes work when you need it.
What is hibernate mode in Windows? Unlike "Sleep" mode, hibernate mode put your session in physical disk (hard disk) and switch off your computer totally. Windows can let you quickly resume your work by loading the session from physical disk without needing to reload the whole operating system.

Enable Hibernate in Windows Vista
Hibernate feature is disabled by default in Windows Vista, to enable hibernate, make sure you have the administrator right. Then open a command prompt (click on Start button and type cmd in the search box) and type in the following command and hit enter:
powercfg -H ON
... And that's all, hibernate feature is enabled. You can check this by clicking Start button and then click on the little right arrow button on the bottom right of the Start menu. You will see the Hibernate option is there.

To Make Hibernate as the Default Action
You may want to make your computer hibernate when you press the power button on your computer or click on the power button on your start menu. The changes will require a few more steps.
First, make sure the powercfg -H ON command is executed in administrator mode. Open control panel and switch to classic mode by clicking "Classic View" at the left hand side of the window. Double-clicks on the "Power Options" icon in the control panel window.

Click on "Choose when to turn off the display" link at the left hand side of the window.

Click on the "Change advance power settings" link.

A window will be popped-up. Expand the "Power buttons and lid" tree. You will see a list of action that you can configure. Click on the action you want to configure and a list of option will be available for you to choose (Do nothing, sleep or hibernate).

Finally, click OK button to accept the changes made.