There shouldn't be a currency imho,
I wonder what people want to buy anyway. It's not like there are stores and stuff in a vault.
Remember that vaults were designed to host a thousand people. Vault was organized like a little town, with weaponry, shops, rooms for each inhabitant...
How technical are the devs? Can you add in things such as a 'food and water requirement' kind of situation? As in...make the vault litterally a mini-sim of ACTUAllY being in a vault and having all the challenges? (Food dispensers break down, fights over items, power struggles, skills being places, 'teams' and 'jobs'