Can you go to vbulletin options --> show all settings and try to save all settings at once, does that work?
If it doesn't please check and verify that your server is running modsecurity, you can verify that its running modsecurity by viewing your php info, it should be listed under apache2handlers Loaded Modules box.
Example:
Code:
mod_env mod_mime mod_security2 mod_negotiation mod_php5 mod_rewrite mod_setenvif mod_status mod_unique_id