"In theory, focusing all the down on the torso area should keep you warmer, since that is where all your major organs are."
Your skin is a major organ, and 90%-95% of all heat is lost through it.
This might help factor into your super-formula for the great vest versus jacket debate. It seems you want to reach that optimal mix for core warmth, and be able to shed unneeded weight.
You have stated that you will be considering the heat while you are only doing light work (tending camp, making food) and not while hiking. You will not be fully at rest, however, so this should factor into our grand equation here.
That "light work" might get tricky, though? If you are a sweaty guy, or in a moist environment, this will increase heat loss.
Your question is also tricky in that your view of heat hasn't been defined as "I am comfortable at this temperature" or "I am just about to enter hypothermia." Are you looking to see what is the optimal down vest/jacket combination to keep you alive (garment weight considered)?
It seems people have been trying to answer your question with their own empirical knowledge, which without knowing more from you, seems the best answers you will get, for now.