Thats why you doctor the archive a bit. Add a headerimage, and some links to other parts of your site. (I like to link to the forums)
vBulletin isnt PHPbb. vBulletin has no problem being spidered by google. This has been discussed and discussed. Many points of view on this subject really.
vbSEO -- is it worth the price ?
Is VBSEO worth the $149.00 cost?
Does vBulletin need SEO mods?