I am often wrong but never in doubt. So I will add some of my new opinions about the vest fabric and garment composition. I am presenting my thinking and reasoning behind it so others can add their own thoughts and make their own decisions. I have no special insight into the actual facts.
Since @
spectreseven has confirmed the front of the vest is nylon I have gone back to the original images to look for additional details. From the highly magnified images I still see the weave of the two front pockets as having a diagonal twill weave. These images are clear enough to determine there is definitely not a grid pattern which is characteristic of a rip-stop fabric.
At the same time I still cannot determine a weave pattern of the vest’s base fabric. The surface is either coated, the filaments are too fine to be individually recognized, or the fabric’s reflective/photographic properties are very different than the pocket fabric. Also between some of the horizontal topstitching (mainly visible on the lower part of the vest under the pockets) there is some indication of additional horizontal artifacts. I added this observation in my own original notes but did not share it as I thought it came from the photographic and magnification processes and not the fabric and its appearance was inconsistent. When they are seen, “broken lines” do seem regular enough that they could be a part of the fabric structure. For example the thicker horizontal threads in rip-stop nylon. However, I cannot find any images which suggest the same type of vertical artifact which would also appear in rip-stop fabric. This could be a product of the lighting. Light washing down from above would highlight any horizontal differences, but not the vertical ones. I am not sure if any of the images have a strong side-lit presentation.
If I look at photos of the storm flap on the left side of the vest, there is some evidence of the same type of horizontal artifact as seen in the base fabric (which I had not noticed before). Also the reflective properties of the flap seem to differ from the pocket on the left side. This could mean they are different colors, different fabrics, or both. EDIT: Any color variation could also come from "weathering"
If the base fabric is (rip-stop?) nylon, then I would suggest that the storm flap is also nylon but the pockets are cotton twill which matches the vest’s back. Another observation which I had made before but will repeat here is that the flap also appears to be padded. When viewed on edge it has a thickness which is greater than two pieces of fabric especially if that fabric is thin nylon. I do not believe it is as thick as the vest itself. I am using three layers of cotton batting for the vest and will probably use two layers for the flap.