Vault Cash

See: U.S. Mint.

Vault cash is cash that banks keep on hand in their vaults.

Banks need some liquidity so that they can cash checks and make withdrawals for banking customers. But banks make money by investing the cash that its customers give it to hold onto (in exchange for some of that investment in small interest rate increments, usually; at least keeping pace with inflation would be nice). So banks are incentivized to invest as much of that money as possible.

The Federal Reserve makes banks keep a certain amount of cash on hand, known as "reserve requirements." While runs on banks don’t happen so much nowadays, this is one of the things that prompted the need for a neutral(ish) banking institution like the Fed, to keep banks in check with the liquidity that they keep on hand.