If it is a wholly owned subsidiary of the bank then it is an asset of the bank. When the FDIC makes their Friday afternoon raids they are entitled to all assets regardless of their corporate structure.
Let’s say WAMU sets up a subsidiary called The Great WAMU Vault Company, Inc. as a California corporation and transferred all of their vaults to the subsidiary. Would this mean that when the FDIC walks through the door with a seizure order they can’t touch the vault?
My guess would be that when a bank signs a deal with the FDIC they pledge all assets of the bank holding company.
I am not an expert in this field, but that’s my take on the question.