Running out of vault cash is not a bank run. Those are two completely different things. A bank run is something that forces the bank out of business -- it make

Running out of vault cash is not a bank run. Those are two completely different things. A bank run is something that forces the bank out of business -- it makes the bank insolvent. This happens because the bank can't convert its illiquid assets (e.g. mortgages) to cash in order to meet depositor outflow. That doesn't happen anymore in part because there are markets for things like mortgages that didn't exist before (making those mortgages liquid) and also in part because banks are on the one hand regulated to control what types of assets they are allowed to hold but at the same time they are given access to lending facilities in which they can pledge their assets as collateral to get reserves directly from the Federal reserve, which will never run out of reserves, and with those reserves they can purchase cash from the Bureau of Engraving. Then trucks will ship that cash to the branch. Yes, it may take some time, but the bank will always be able to close out deposits after a brief delay. Obviously banks try to minimize their cash holdings because cash pays no interest. So they have cash management professionals whose job is to predict cash demand (which is predictable) and hold no more cash than is necessary to meet that demand, parking the rest of their working capital into short term interest bearing liabilities like commercial paper or bills. Today, even reserves pay interest. So everything is better than cash. That necessarily means unexpected events will lead to a situation in which the cash management team underestimates a cash need and so they have to sell some assets (or borrow reserves), pick up the phone and order a truck. That happens quickly but not instantly. When that happens, e.g. when the cash management team makes a mistake, it's not considered a bank run, even if it means that a branch has to close until the armored trucks arrive. It's no different than Safeway running out of apples -- actually there can be prolonged apple shortages at safeway, but there can be no prolonged cash shortages at a bank, since the bureau of engraving can print cash with much lower latency than farmers can grow more apples.