No problem… Basically I know very little about vending machines but I did some research years ago that lead me to believe there is a standard protocol for communicating data between the bill validator and the vending machine control system… that’s where I would focus… or by removing the control system completely and inserting your own “brain” into it… it would be difficult… you’d have to deal with power electronics of course… switching large currents and controlling vending motors and getting position sensor data and position switch data from the mechanical components within the vending machine… but it would give the highest reward.
The Pi is just a computer… it has no reader built int… so you would need to interface one. You could use the I/O pins and tie in the PN532, but doing so would be difficult compared to simply plugging in an ACR122U and loading the drivers for Linux.