Correct me if I’m wrong, but cash cards should only work if isuing bank processing centre (and on the top tier level processing centre of Visa/Mastercard/AmEx etc…) is still working. Which, I assume, according to overall cataclysm, isnt. Maybe there are some variation with crads with EMV-chip, and their ability to support offline authorisation but I dont know for sure.
Useful to mention about lack of power for ATM-s, either.
So they have to be little less than chunks of plastic.
As for vending machines (powered from some source) - I support the idea, and it would be useful to operate them via cash (and to merge cash in game, if it isnt there already). Or to break them loot (if they are not too sturdy)