I am using Web Wiz Forums and I have this in robots.txt:
User-agent: *
Disallow: /forum/search_form.asp
Disallow: /forum/registration_rules.asp
Disallow: /forum/login_user.asp
Disallow: /forum/members.asp
Disallow: /forum/post_message_form.asp
But for vbulletin you can check this :
http://www.blogstorm.co.uk/blog/seo-...lletin-forums/