Recently I ran into an issue where I could not log into vBulletin (after changing the forum URL from (example.com/forums/ to forums.example.com). Lesson learned – you need to clear the cookie path in the admin before changing over to the new URL.
If it is too late though, you can download a file called “tools.php” that is in the do_not_upload file of the standard vBulletin folder. Upload that to the admincp folder along with the “install” folder and then go to your forum’s URL /admincp/tools.php. You will be prompted for your customer ID, and then you can reset your cookie settings.
Once you are done, make sure to delete the file and install folder. Login should once again work!