[kh99]

That's happening because for some reason vbulletin thinks you have -6 unread messages (it's not expecting negative numbers, so it gets displayed as a large number). You might be able to fix it by going to your inbox, selecting some messages, then in the "Selected Messages" menu at the bottom choose "Mark as Read" (it doesn't matter which messages you choose or how many, you just need to select at least one to make it recalculate).

Edit: oops, I was thinking private messages, obviously the above fix won't work for visitor messages.