[encryption]

How have you setup vbulletin to send emails, using php mail () function or via SMTP ?

if using php mail () function and you are running WHM / CPanel, login to WHM and under Server Configuration >> Tweak Settings, make sure the following box is unchecked

"Prevent the user "nobody" from sending out mail to remote addresses (PHP and CGI scripts generally run as nobody if you are not using PHPSuexec and Suexec respectively.)"

Secondly, (assuming you are running WHM / CPanel), SSH into the server and run

/scripts/eximup --force

See what that does.

Finally, create a test email account under your domain, login to your webmail and send yourself an email (to your gmail/yahoo/hotmail account) and see if you get it. If not and the email bounces back, paste the failed mail error here.