When the VIP mailbox is selected, is there an item in the menus somewhere that will start the delete process? or an item in the bottom of the Mail window, where you can add new smart folders, and also remove them.
What happens if you select the VIP box, and simply press Delete.
Is there a setting in the Mail preferences that allow you edit or delete a VIP mailbox?
If you go to Mail's help menu, type in 'Remove VIP mailbox' - do you get any results there?
As you can tell, I don't have Mountain Lion yet, so trying to supply some hints on some things to try, or how to get help from within the Mail app.