I used to manage the vending contract for a national grocery chain. (230 stores, maybe 800-900 machines).
Some things to consider:
1) Kids and parents like the ones that dispense a small toy instead of candy all the time. Many sales were due to the kids, so items that kids find fun are good. ... toys, gumballs, jawbreakers, etc. The game / toy machines (that always dispense a small prize) are also fun, even the ones with a crazy coin drop for a gumball.
2) For some reason nut / peanut machines sell well, especially small ones in maintenance waiting rooms where men work (I was told) BUT peanut allergies and sanitary condition must be considered, and these were banned from our stores unless nuts were packaged.
3) Half of our sales seemed to be from employees.! Machines for offices and school areas (think a tutoring center or music lesson area) were good sellers. So not just waiting areas, but places where someone wants a snack, but the employee size is too small for a full size vending machine... and a $1 coin machine works. -- car repair shops, truck driver areas, Lordco auto parts, etc.
4) We required machines to have a complete refurbishment (new paint, condition, etc) every 5 years, which can be expensive. The machines must look great to be placed easily.
5) Other than small candy / coin machines, the larger vending styles here need to covert to card systems.. this is a huge trend, that people don't carry cash. Also, I went through the transition where the government brought out new money formats, so the dollar readers all had to be upgraded to work, or coin slots changed to allow for coin sizes, etc.
6)My predecessor was so aggressive in negotiation the placement contract and terms, and the vendors were outbidding each other, to the point that one went broke within 18 months of getting the 3 year contract, and I had to go out to get new vendors after. Looking at the original contract, I don't think any business could have made money on it.