Might be able to change the qty in the cart. Otherwise might have to call in an order. Might be set that way to prevent people buying all the stock to turn around and resell. Purchases from the Vault are intended for personal use and not to be resold.
I'd try giving them a call. Sometimes websites with high priced items(such as Oakley), you can only buy a limited quantity to prevent fraud(i.e buying items with stolen credit cards to resell), or as GRF said, just to resell in general since Oakley is strict about resellers.