One bug that I've noticed with the vault of piety is when you purchase items at any shop that involves a number slider, for example buying 20 potions or injury kits, the vault of piety will keep track of the last number selected, without showing a slider.
For example if you buy health potions, it will ask you how many. Let's say you choose to buy 10. When you enter the vault of piety, it will try to purchase 10 of those potions at once also. If you can't afford 10 then it will simply say you were unable to purchase.
Try going back to a shop and resetting the slider by purchasing a single potion or injury kit, then try in the vault of piety again. That seemed to solve the problem for me.
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For example if you buy health potions, it will ask you how many. Let's say you choose to buy 10. When you enter the vault of piety, it will try to purchase 10 of those potions at once also. If you can't afford 10 then it will simply say you were unable to purchase.
Try going back to a shop and resetting the slider by purchasing a single potion or injury kit, then try in the vault of piety again. That seemed to solve the problem for me.