Expected warmth from combining down vest and down jacket

Depending on size… but if you can, I think you should layer your jacket over your vest for more combo efficiency that focuses on the core.

A buddy of mine does it all the time and gets on well with it.

I agree w/ Ben, if possible (ie not loss of loft from compression), I think the vest would be more efficient under the jacket (you lose heat out of the arm holes)

as long is there is no loss of loft I think 20 degrees w/ the layers you list is reasonable (I have a MB Alpine Light Parka and Ex-Light vest w/ similar layers and have been fine @ 20)- I run a little warm, if you run the opposite, then you might need a little bump

"Basically, I'm curious how low I can push this system while being inactive in camp and still remain warm."
Not something someone else can answer. I could "push that system" well below 0F.
My Alpine Light Down Parka is enough for me at that temp.
'Course I met an older gentleman *hiking* in a similar jacket with two shirts and a heavy fleece jacket under it Thursday morning when I was just wearing a Capilene 2 top, so maybe I'm not the best one to ask(it was over 40F!!!).

I've done it once – so far – and it worked very well. It was with my EB Down Light jacket (see my avatar) and my Xmas gift EB Down Tech vest under. At 8,000+ ft. I was very comfy on a windy 15 F. day.

Having both collars up was also nice to keep out drafts.