I have a friend who has many vending machines, everything from chocolates to coffee to prepacked sandwiches, bubble gum. I remember him telling me that it is hard to tell if a machine is going to do well until you actually place it.
Hospitals he said are the better place for the prepacked sandwiches etc. but require a lot of work to keep the products fresh
The ones he has in shopping centres are usually chocolates or game type machine he said these take in a lot of cash but it also costs him a lot in floor space, rent.

He does nothing else, as far as how much he makes out of them he wouldnt tell me cause he was too busy concentrating on driving his brand new ski boat.