all the vests were apparently made from poly-pro twill, also referred to as tackle twill, which is a 100% polyester twill fabric.... which in a very real sense IS just like an outdoor windbreaker material as a lot of outdoor jackets/coats use polyester or polyester/nylon blends in their shells. It is also interesting to note that the vest was actually referred to as a "jacket" in the wardrobe breakdown sheet for the Fett costume list, not a vest.
So the shine is from polyester, which can have a wide variety of sheen depending on the production process. It can be very shiny or very dull, but when weathered and photographed with just the right lighting, it can mimic the shine of leather like it does in some of the pp2 photos. But it's not actually leather.