This, for example, would be disallowed by the vBulletin template system, as it contains a call to a 'forbidden' function: mysql_query.
At the time of writing, the list of allowed 'safe' functions is as follows:
<vb:if condition="$my_variable = mysql_query('SELECT * FROM mytable')"> <!-- naughty naughty... --> </vb:if>
This is an example of 'safe' functions being used in a template conditional:
Functions marked * are custom functions defined by vBulletin itself. Each function name is a link that will take you to the documentation for that function. Use of these functions requires knowledge of PHP
<vb:if condition="isset($my_variable) AND is_browser('ie')"> <!-- $my_variable is set and the browser is Internet Explorer --> </vb:if>