If you are experiencing some display issues in IE8 with your vBulletin-based forums (and/or general sites), you might want to try adding the following meta-tag in your HEAD section:
<head>
...
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7"/>
...
</head>
You may get more info about this meta-tag and issues that might be connected with IE8’s strictness at the IEBlog, and/or via this Microsoft Support article.