All we know this simple procedure.vB4, go into admincp(www.yoursite.com/admincp) That is assuming you have your forum in the root of the domain.
Go to vBulletin Options, scroll down to "Human Verification Options". Use that to fix your problem. Then make sure in Usergroup Manager that you edit the "not logged in" usergroup and scroll down to "Require Human Verification on "Configured Actions"... Edit the check boxes and you should be good to go.
You know it's something unique thus bot will stuck when try to login automatically.Why would you need captcha login anyways? Just curious. If you use it for registration, how would they have an account to login to?
URL please...Did you try advanced login pludin?
http://www.vbulletin.org/forum/showthread.php?t=147601