http://www.vbulletin.org/forum/forumdisplay.php?f=198 vbulletin module, hack, plug-in, etc, etc. We have a new service we are looking to create a vbulletin plugin for.
A "module", "hack", and "plugin" can mean very different things in the vBulletin world. If you narrow down what type of work you want done it'd be better.
Upon registration of a new forum user, the details are sent back to our servers for validation, if a negative response is given, the users registration is rejected. Upon a banning of a user, that is reported to our server.