Open your config.php in your includes folder and look for this:
PHP Code:
// Prefix that all vBulletin cookies will have
// For example
$cookieprefix = '/';
Change your cookie prefix in coppermine to what you have your vb cookie prefix set to. That should do the trick.
If you don't have your cookie prefix set in your config.php then set it to whatever you have set in your vbulletin options and make sure that coppermine's cookie prefix matches!
Bigwrenn