The more walkable the area the more likely someone will use your vending machine.
Or areas where multiple people are "trapped" with few options. Think waiting rooms of any kind...automotive dealerships, repair shops, medical clinics, etc.
Where I live there is always a wait at the DMV and they are private contractors to the state, not state run. Being small businesses themselves they may go for the added income of leasing space to vending machines.
I fully believe self service is the way of the future. Look at the way stores are going with self check out. Back in the early 1900's there were automat diners. I can see this making a comeback. Starting small with a few vending machines and scaling up to be the pioneer that brings back the automat wouldn't be the worst idea I've ever heard.