Actually, upon further inspection, it seems to be a standard part of Vbulletin code?
The sites (not mine) that I looked at all have this. In fact VB.org has this if you view page source their home page.
Even your forum has this!:
PHP Code:
<meta http-equiv="Cache-Control" content="no-cache" />
<meta http-equiv="Pragma" content="no-cache" />
<meta http-equiv="Expires" content="0" />
Granted it differs from mine in that the value "0" is present instead of "-1" (no idea what that means)
Edit: Further more reading, this is hardcoded into the
forum home of Vbulletin, which explains that. It is not present for example, on a random forum page on my forum.
So is this really the problem?