I don't do this myself, but I can't imagine this is in any way low effort. If you are the only person working, you are going to do a lot of leg work, customer service, networking, and physical labor of moving and then restocking machines regularly. And then if you employ people, well, now you're running a fairly labor intensive business.
It probably is big bucks (or has the potential to be), but I'm sure it's a lot of work.
Ok I know for sure it's big bucks, because my girlfriend does vending machine layouts, for lack of a better term, for the company she works for. But that's for industrial products like gloves, knives, dust masks, etc, not traditional vending products you might be thinking of. And they do make a lot of money off it. But they also have an army of employees around the country who are constantly pushing the machines and trying to get them in new locations.