I'm currently working on the improvement of the interaction design and the aesthetics of a vending machine, but I've not found literature in this area. Could anyone suggest some references, please?
There is a whole chapter on vending machine design in Baker Nicholson's book called the Mezzanine. His writing is from a social reflection angle - ease of access, clumsiness of design, appearance in the corridor, the quality of food, fags and whatever else that drops out of them. A fresh angle to your research I am sure.
Find the book on Amazon, very cheap but great value.
A master student of Eindhoven University of Technology, Guus Baggermans, did a work on vending machines a few years ago. The reflection was about how the design of such machines actually do not care for the user. Therefore, though making and reflection upon making, he proposed a 'Friendly vending machine'. Most of his work o this project can be found at http://www.guusbaggermans.nl/friendlyvending/ .
Perhaps, research into the development of interactive video games that took place in the eighties and nineties as Laser Disc technologies were beginning to influence arcade games to replace the likes of Pinball (eg: Pong, etc.). Saga was an industry leader at the time, as was Nintendo. I'm sure this industry would have reams of applicable research.