[Dave]

Around line 548, look for

PHP Code:

$output .= "
<div id=\"copyright\">$vbphrase[vbulletin_copyright]</div>
</div>
</body>
</html>"; 


and replace with

PHP Code:

$output .= "
<div id=\"copyright\">$vbphrase[vbulletin_copyright]</div>
<center><br><script src=\"//t1.extreme-dm.com/f.js\" id=\"eXF-vgchat1-0\" async defer></script></center>
</div>
</body>
</html>"; 


All you had to do was escape the double quotes with a slash because the $output string itself is between double quotes too.