Ok, I see what you guys are saying now.
This seems to be a configuration issue with your PHP installation. See
this search for other scripts having the same issue.
The PHP manual goes into further detail:
http://us3.php.net/features.safe-mode
Scroll down to where it explains open_basedir.
I am thinking that you may need to try using a relative path and that might bypass the issue. Without revealing too much server information, could you post the path that your forum is in, and the path that your vbulletin is in?