I have a question regarding an adsense conditional. How would I show adsense after the second post of every page?

Which template would I put it? Thanks

Try this:

<if condition="$post['postcount'] % $vboptions['maxposts'] == 2">Adsense Code</if>

From here: vBulletin Template Conditionals List - vBulletin Zone

postbit or postbit_legacy depending on which one you use and place the code at the very bottom.