Started up a new lone wanderer and i was wondering this as I left the vault. Were caps used in Vault 101? There's no dialogue prior to leaving, or when arriving at Megaton, that you have to pay for things with caps. Trying to remember if there is any explanation anywhere in FO3, for the use of caps. I prefer paying with pre-war money myself

We're not given that explanation, though the closest we come to is bargaining with Moriarty. He doesn't say "we use bottlecaps as currency" but "I'll tell you where your father went for 100 caps. If you don't have that much, I have some work you can do that should give you juuust enough caps to pay me with."

It shouldn't take a genius after that whole runabout (and maybe after an encounter with a trader) to realize that bottlecaps seem to be used as currency.

I don′t think anyone needed currency in the vaults. Everyone was working to help each other and the need for profit just wasn′t there. Trading goods and services was the way to go, me reckons.