You could link the Favicon in the "headerinclude" template.
Before the closing
</head> tag:
Code:
<link rel="icon" type="image/png" href="http://example.com/favicon.png">
OR
If your Favicon is in the same directory as your vBulletin forum:
Code:
<link rel="icon" type="image/png" href="favicon.png">
Change the
RED with where your Favicon is located. You can use the full URL or the local directory. You can also use ICO or PNG format for your Favicon. This is how I have my Favicon set on my vB forum.