There is a post related to the issue you describe on the vBulletin forums.
Execute the following SQL query:
UPDATE PREFIX_setting SET value = "" WHERE varname = "banip";
Replace the PREFIX value by yours - in case you use it. Otherwise, remove PREFIX_ from the command.
That SQL query should remove all IPs banned. I suggest you create a database backup before executing that command or making any changes.
In case that doesn't help, there are other steps you can try by following this vBulletin thread.
