On vBulletin, with vBookie installed, it is not possible to donate a negative amount of VChips to yourself or another user; however is it possible to alter the input string to get around this limitation and donate any amount you want to your account, or that of another registered user.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/44641/vBookie.txt
Source: https://packetstormsecurity.com/files/44641/vBookie.txt.html