From my limited experience, angler pressure is the overriding factor with these fish rather than time of day. They are big trout and will hammer almost anything if not already spooked. On the few occasions I have fished water that has had no angling pressure whatsoever, I have caught fish at any time of the day. In the absence of angling pressure, many other factors such as, water clarity, temperature, ambient light, etc, etc. will have a greater influence than the time of day (except that the time of day may obviously influence things such as temperature, light etc).
So if you are fishing the Vedder, first light is one of many many variables that will probably help to increase your chances, but if you are in the back of beyond where fish hardly ever see an angler, you can enjoy those extra hours in bed without feeling you are missing out.