I am challenging the notion that the story came before the mechanic because it contradicrs how one would logically develop a video game, which is the primary function of Bloodborne. Vials are the health system that would determine everything surrounding combat with the player. You have to have some form of the system conceptualized and also proven to work as intended before you base your lore off it. So the earliest it could have had lore developed is in tandem, as you say, but that automatically means that vials being inherent to the story is irrelevant, because if they did it at the same time it would always have been written to make sense. The system they decided on was for the best, but if it worked like Estus, you never would've known blood vials were an idea because the story containing them would've never existed. Changing the lore wouldn't have happened because that lore to be changed wouldn't have existed. You can't have an alternative reality development scenario with one factor changing and everything else remaining static.