Is safe_mode on?
admincp --> maintenance --> view php info
Look for safe_mode on/off, let us know what its set to
If its on, use the option in the admincp to set a directory to allow uploads in a safe manner to not harm the server as its a vhost.
admincp --> vbulletin options --> Server Settings and Optimization Options
Safe Mode Upload Enabled <-- set to yes
Safe Mode Temporary Directory /temp <-- this is relative to your public_html/root folder.
Ie...
http://www.mydomain.com/forum/temp and chmod the new temp folder to 777