your site.com/threads

your site.com/forums

your site.com/whats-new

your site.com/tags

your site.com/help

RewriteEngine On
RewriteRule ^threads/(.*)$ /vb/threads/$1 [R=301,L]
RewriteRule ^forums/(.*)$ /vb/forums/$1 [R=301,L]
RewriteRule ^members/(.*)$ /vb/members/$1 [R=301,L]
RewriteRule ^whats-new/(.*)$ /vb/whats-new/$1 [R=301,L]
RewriteRule ^tags/(.*)$ /vb/tags/$1 [R=301,L]
RewriteRule ^help/(.*)$ /vb/help/$1 [R=301,L]