You can enable vibrate mode in Windows Phone by following the below steps.
How to enable or disable Vibrate mode in Windows Phone 7 ?
1. Tap / Click right arrow in the start screen to navigate to the list of Apps .
2. Tap / Click the Settings in the Application list
3. Tap / click the Ringtones + Sounds option to launch the Settings Form
4. Toggle / Switch on the Vibrate option to enable Vibrate mode . Make sure that the Toggle Button is Green Color for enabling the Vibrate Option .