New users not found in admincp...

[Thunderbird8]

Spambots usually have the resources to get through the email confirmation, since that's a rather common setting on many boards. Since you're moderating new users, you'll likely find them in the Users Awaiting Moderation usergroup, if they aren't in the email confirmation usergroup.

Granted, I've never converted to vB from another board, so it's possible that the issue might be related to that.