I've seen it reported as us owning 50% of Vela's rights, but a sell on clause makes much more sense. Not even sure English clubs are allowed to co-own players.
Question is, would it have been moral to buy him for the £3.5m buy-out figure and immediately sell him to another club for £20m? Would Vela have even agreed to that? Probably not. Think we've got about as much as we could out of him.
Of course, if, unlike me, you wanted him here as part of the squad then you'll be disappointed.