This is an interesting item. Its best used during rock climbing and or in combination with item transportation between bases. Allowing you to carry more gear between trips. Its very low weight (0.25 kg // 0.55lbs) can also be helpful for the new rock climbing mechanic. Down Vest is vastly superior to the insulated vest (Grey in color) . Insulated Vest is heavier, offers no wind protection, and decays faster over time. The down Vest does decay quickly in comparison to other coats, but less quickly then its grey colored counterpart. Consider using vests during the early stages of sandbox when your focus is entirely on item collection and transportation between your safehouses.
For hunting Down Vest simply does not provide enough warmth. There are other lightweight options in the game which are simply better suited for that task.