Waterstones source their books from Gardners, we do not have a direct relationship with them so would not have any control or be able to guarantee listing on their site / stocking in their stores.
The best way to increase chances of this being available is to ensure Bookvault are listed as the distributor with Neilsen, Gardners would then be able to purchase the books from us, and in turn, Waterstones order from them.
As both Gardners and Waterstones would be involved in the sales chain, the available discount you supply may need to be higher.