https://vborg.vbsupport.ru/showthread.php?t=62164 - How to create your own vBulletin-powered page!
I used this method, then iframed the actual chat on the vbulletin powered page in the template like this:
HTML Code:
$stylevar[htmldoctype]
<html dir="$stylevar[textdirection]" lang="$stylevar[languagecode]">
<head>
<title>$vboptions[bbtitle]</title>
$headinclude
</head>
<body>
$header
$navbar
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" align="center">
<tr>
<td class="tcat">Your Chat Title</td>
</tr>
<tr>
<td class="alt1"><div align="center"><iframe src="$vboptions[bburl]/chat/index.php" width="100%" height="600" frameborder="0" marginwidth="0" marginheight="0" scrolling="no"></iframe></div></td>
</tr>
</table>
$footer
</body>
</html>