IE9 will try to switch to "Compatibility Mode" in some cases with some sites (it's turned on by default; you have to manually turn it off, but there's an alternative --read further). I've had this problem in the past (well, 95% of Web Developers in the world have had a problem with IE since its inception; IE is a curseword in most circles).
Anyway, you may want to look at the CSS (the "skins") and see if it has some "specific" IE6/IE7 entries and I'd do a Google Search on how to tweak those for IE9 (usually the ones for IE8 work).
Also, inside the Head tags, along with the Meta tags, I'd add this (it will have to go in each of the live Header file for your template/skin or whatever the setup is in vBulletin):
I don't see it in your site right now. I recommend you try this.