Bill works for Gringotts, and he's apparently highly respected, even amongst the goblins. If Bill were to report that they had one of the Dark Lord's horcruxes in one of their vaults, what would be their response? It's a MASSIVE security breach, if nothing else, given what those are and what they can do. Hell, the diary was sapient and could interact with the outside world in a few ways, including casting spells. Would the goblins see the sanctity of their deals above the security of their vaults, or the other way around? Because I could see them getting absolutely pissed that one of their clients is storing such volatile, dangerous, and insecure items in their vault. It's almost assuredly against goblin law, as well as whatever contract was signed to open the vault, and it wouldn't surprise me if they brought in their best and brightest to secure, test, and destroy the thing, and likely confiscate the rest of the vault entirely as a fine. And depending on the method of destruction, it could cause Moldyfarts some serious problems. Sympathetic magic, anyone?