IE8 was created to be compliant with modern HTML and CSS standards. However, this caused older sites to "break" because those sites were coded to work with the idiosyncrasies of previous versions of IE. Microsoft created Compatibility Mode (called Compatibility View within the IE8 browser) to force newer IE versions to display older sites using the appropriate version of the rendering engine. Conversely, Compatibility Mode is also useful for signalling that your site has been updated to be fully compatible with current versions of IE.
Compatibility Mode enables Internet Explorer 8 and later versions of IE to correctly view sites coded with "hacks" to accommodate earlier versions. If your vBulletin forums display correctly only in certain versions of IE, you have several options for triggering Compatibility Mode for members and other visitors using IE8 and later.
Depending on when you created your vBulletin site and which template you're using, some of the HTML or CSS code may be designed for compatibility with older or newer versions of IE. If you're noticing a problem such as disappearing text or objects that are out of position in the newer browser, compatibility may be an issue. If you notice the "broken page" icon in IE8 when you view your site, click the icon to see your site in Compatibility View, and if it looks correct you should add a compatibility header to your site.
There are two primary methods for changing the Compatibility Mode of your vBulletin forums. First toggle the option in the vBulletin Administration panel by clicking Options on the right-hand side of the page, then choosing Cookies and HTTP Header Options in vBulletin 4 or later. At the bottom of the page, toggle the Force Latest IE Mode radio button and click "Save." Refresh your site in the browser to see if the problem persists.
Manually Adding a Header
If you're seeing the "broken page" icon in IE8 even when the Force Latest IE Mode option is set to No, try adding the following <meta> tag to the header of your pages, above every other tag but the <title> and other <meta> tags:
<meta http-equiv="X-UA-Compatible" content="IE=7" />
This tag will force IE8 and later to display your pages using the IE7 rendering engine. Over time you should update any elements coded for earlier versions of IE so that they comply with current standards and display consistently in all browsers.