For me, the oddest vending machine area I've found in Japan is a porn vending machine area open to all. The machines were labeled with signs saying you had to be 18 years old to buy but certainly nothing was enforcing that. The location of the one I remember is about 600m from Okubo station (Tokyo) in an obscure location at the side of what appeared to be a wholesale food market area. Sorry, I didn't take photos but could do so the next time I am there if there is interest, provided it is still there. As I found the location online first (despite having very limited Japanese skills), it may be that another diligent searcher could still also do the same.

And no, I didn't buy anything!