It would be
Code:
<if condition="stristr($post['haystack'],'needle')"></if>
, except that stristr is not on the list of functions allowed.
http://www.vbulletin.com/docs/html/m...n_conditionals
You can add to the list of allowed functions in a plugin, like in the attached example, however, it's best to first see if you can do the same thing with an already allowed function.