My interpretation of the image tells me it is a u-neck vest. with buckles instead of buttons. I don't think it has a collar, I think what you are seeing is the collar from the shirt worn under the vest. The colors used on the collar are closer to the colors of the shirt on the lower back beneath the vest. The vest has more red in it in my opinion.
Looks cloth to me also, as leather would not have the kind of drape that the artists created in this picture. specifically I would site the wrinkle that goes from the back of the lower buckle up to the middle of the back. It is a two panel back and all the seams are dressed with a contrasting bias tape (like you see on the edge of a blanket. Not common, but it is sometimes used as trim.
Now the buckles are wrong. we see the loose end of the straps, but we also see the buckle. if you think about a belt, the buckle is one end and the loose end is the other. there would have to be another buckle on the other side, and for style reasons I would add a third buckle near the right side seem to balance the extra one at the right side seam.
There is some type of piping along the arm holes which could be done with either a braided or rope cord type piping, or you could make a solid cloth piping that has a spiral stitch of the light color (or metallic) thread as an accent.
I think I might have gone a little over the edge, but to answer your original question, I think you need to start looking at a U-Neck vest as your starting point, but because you are creating something that was a painting and not based on a real garment, what ever interpretation you use will probably be right. My advice, make it your own.
Joshbot