Guild Vault Voucher (7th Slot)

Guild Vault Voucher (7th Slot) is an epic guild vendor item that allows for the opening of the 7th slot for your guild's vault. Beware, while non-guild master members can purchase this item, they cannot use it and they cannot trade it to their guild masters as it binds on purchase.

Source
This item is sold by the following vendors for after completing the achievement :

You will also need to purchase this item.