Modify Ominous Vault loot

I guess this should be a simple question but nor Google nor ChatGPT made anything clear. I did manage to make custom loot tables for chests or when breaking a block, but not for vaults since its some sort of "using a key" think. Even tried asking ChatGPT for a simpler question like "how to make pumpkins drop diamonds when sheared" but started saying stuff about some advancement thing.
I just want the vault to drop shulker shells