The default meta tags are:

<meta name="keywords" content="The Front Page, vBulletin 4.0, CMS" />
<meta name="description" content="vBulletin 4.0 Publishing Suite with CMS" />

To edit this info:
1. Go to your CMS Homepage
2. Mouseover the top section title
3. Click on the pencil
4. Edit info on bottom right column