The vraydomelight is just one big light really, and happens to have a sun in it. So I guess what you are seeing is less sharp shadows from the hdr sun but then there are more defined shadows from the rest of the sky, whereas the vraysky is just contributing to GI. If you look at the vraylighting element on one of the vraysun renders and compare it to the vraylighting element from a hdr sky render you can see what i mean. Correct me if i am wrong someone!