While it may have been on another version of tracfone, companies often do change their programming features for each phone they have. Your manual which may be generic and describes where and how the vibrate function works on this make and model of phone may not have the feature enabled. If it's not there you cannot, in all likelihood, turn this feature on.
The vibration option can be used to notify you of new calls, messages and warnings. Press the menu button on your phone and locate the settings icon. After opening the settings page scroll to the ring style option then select 'options', 'sounds', and save your changes as set.