The biggest problem I have, and I hate to say it, but there is no government agency to guarantee the deposit so there is the very real chance that if the bank makes too many bad loans they will go under and lose the depositor's money. Even if you can get the loans done in the proper legal form, which I'm not sure of at the moment, then you have the issue of what happens when the value of bitcoins doubles or triples in a few months? Everyone who borrowed money will now owe two or three times as much, and most will not be able to pay the loan back. So the bank loses out big time on bitcoin appreciation, but on the other hand if the value halves the bank doesn't get any benefit. It only helps the borrower since the bank is only making a profit off the spread in interest between what is charged to the borrower and what is paid to the depositor. So the bank loses when bitcoin goes up, but fails to gain when it goes down.
Governments don't guarantee deposits, the taxpayers do. Take a look at the bill the FDIC (Federal Deposit Insurance Corporation) has racked up so far this year on the US taxpayers behalf, it's more than 8 billion in the red.
This actually does have a solution - The bank doesn't make bad loans, because nobody is there to save it so it uses actual risk management rather than the mickey mouse parade you see in financials today. Theres no law saying banks have to loan money to people who probably won't pay it back, that only happens precisely because we have things like mandatory deposit insurance through the FDIC.
And if The Vault (like any bank) ever started making bad loans and losing money, people would stop making timed deposits with them. But even if that happened, the people with daily accounts would be completely unaffected by this (again this relies on transparency, which would need to be built into the concept)