Paul, every day there are new kids coming online that think they can make a fortune by collecting email addresses and selling them to spammers (multi marketing specialists). Many forums do not allow visitors to see email addresses of members, but some allow their members to view email addresses of other members. Therefore you will always have a small but steady stream of new 'members' who have no intention of doing anything but attempting to harvest email addresses of members. The fact that vBulletin forums don't allow that information to be shared is irrelevent, those kids don't know that until after they try it a few times.
There is no good reason to even allow these kids to complete their registration. They have no intention of ever participating.