[HM666]

Have you overwritten the files with the vBulletin files downloaded from the members area? This is what you need to do to get rid of this problem for now. To my knowledge there is no vblogin.php file in the official download, its called login.php if I remember correctly. So as said before they modified this to use that file.

To find how they got in is a different matter. If you are running your forum on a shared server then that is more and likely how. Shared servers and just that...shared and less secure than a VPS or dedicated server. You can try and speak with your web host and see if they have any way to tell where the attack came from. Most likely the hacker gained access to your FTP and changed/uploaded files to your site.