I have both version of vaults, the object (that takes up space, right) and the door that allows you to equip the vault as you like including collectives etc. On a community lot I have a bank with vaults in th basement. 6 semi-criminal sims have one vault each in order to keep money out of their household net worth and lower their bills. Uhm... I had a fun situation where one bank vault owner died without making needed arrangements, and the heir had to break into the vault to get his heritage :)
EDIT: Lowering bills requires the vault is on another lot. Values in vault on home lot counts towards bills.