bitcoin vault service run by computer security experts would probably only need need 1/10th of the employees as an equivalent sized bank, and it would probably offer superiour security at that.
But they won't have the experience jumping through all the banking system regulatory hoops, or experience dealing with currencies other than bitcoin.
I expect lots of successful bitcoin vault services, and I expect one or more of them to be purchased by one or more banks and made a part of the banks' money services business. If they see bitcoin vault services making big profits, they'll want to own one.
My predictions are frequently wrong, though. Maybe US banks will fight bitcoin tooth-and-nail, or maybe the legal issues will be too unclear for any bank to risk getting involved.