I did find a little more about the Philadelphia Federal Reserve vault. In an application to the National Register of Historic Places (https://www.dot7.state.pa.us/ce_imag...001437_01H.pdf) it is stated there are two vaults in the basement:
The North Vault (1921) consists of 8 individual cells, each once held a different denomination of uncirculated Reserve currency. Its door weights (sic) 51 tons and its steel-rail reinforced walls are 4 feet thick. This vault lies beneath the 1919 section of the building.
The South Vault (1935) lies beneath the oldest section of the building and is built of a better grade concrete with 3 foot thick walls. Its entrance is of the cylinder type, and when closed, it forms a vacuum inside. The vault plus its housing weigh 80 tons and is in the form of a suspended chamber encircled by a narrow walkway. Prisms allow security personnel to look beneath and around it.