After the previous comic, multiple people suggested golden bottle caps as the currency.
I'm not doing that, as the time to make that joke has past, but this was my immediate response when that was first suggested:
"That's it. That's the joke I should have done."
I might have a golden bottle cap show up later in honor of it, however.
I never really thought about it much before now, but from what little I've seen of (functioning) vaults (complex underground bunkers) in Fallout games, they're more communist than capitalist. I'm not sure money even matters within them unless trading with the outside, and jobs are assigned by the people in charge.
This is somewhat ironic given the prewar conflict and the capitalist vs communist propaganda. "American capitalists live on! ...In underground communist vaults."