Medium, usually a medium is a 15-15 1/2 inch neck. These shirts are not custom made to fit. Actually, they are quite baggy. Arm will more than likely be 33-34 in length. And with that small waist of yours the shirt will not fit snug. But if your looking to wear a tie with it you need a medium to fit your neck size.
You honestly could wear a small. It really depends on your built. I'm 175 pounds and I can almost fit into a small. The chest is just a little snug. I would try to find a store that carries them and try them on.
I'm a 15.5 neck, 40R suit and I am wearing a size small as I type this. Much like RL they have different fits/cuts for different shirts. This small is actually one of my best fitting shirts, whereas I have a second BD from them that is a medium and I clearly could not manage a small in that cut.
Go to a VV store or Nordstrom's and try a couple on.
i am 5'9" and 175 athletic build and i have shirts in small from them. i ordered a medium eliot shirt from them once and was swimming in it. strangely i have a striped oxford medium from them that fits fine. VV is generally overpriced for what you get