The only kind of vending machine low maintenance enough that I think it can count as passive is digital vending machines that only take electronic payment, e.g.

The only kind of vending machine low maintenance enough that I think it can count as passive is digital vending machines that only take electronic payment, e.g. a Bitcoin ATM. There's no physical goods to restock so once it's in place, you're hopefully good for a long time. But as you point out, even getting the location, the machine, and then installing it, is enough work to potentially not fully count as passive.