You can use PHP, but the template manager restricts what PHP you can use to these functions:
- in_array
- is_array
- is_numeric
- isset
- empty
- defined
- array
- can_moderate
- can_moderate_calendar
- exec_switch_bg
- is_browser
- is_member_of
http://www.vbulletin.com/docs/html/f...n_conditionals
If you need something other than that, you'll have to do what Dismounted said and use a plugin.