Hi
It's not that hard:
Templates are called with something like $foo = fetch_template(foo), which stores the template in the var $foo. Afterwards, print_output() outputs the whole thing, using the template indicated in the fetch_template() inside print_output().
Additionally, you have template conditionals, with which you can show or hide certain parts of the template based on conditionals. Those look like this:
<if condition="$foo == 2">foo is 2<else />foo isn't 2</if>
The else part is optional.
There's also a lot more information in the vBulletin documentation which you can find at
www.vbulletin.com/docs/
Have fun with vBulletin!