Generally we would avoid doing the redirect unless it is absolutely necessary. So I'd suggest you rather move your vBulletin installation to your /public_html root, saying from experience having the vBulletin in root and using the main domain seems to yield better seo results that redirecting this, sub domain that.
Anyway, back to the redirection, I guess you you like, you could use a simple 301 redirect using .htaccess since thats a good way of preserving your current Search Engine rankings..
Something like this should do it....
Code:
Options +FollowSymLinks
RewriteEngine on
RewriteRule (.*) http://www.yourwebsite.com/$1 [R=301,L]