If somebody put a gun to my head and forced me to pick only one mid-layer for the rest of my camping life, I'd take the Montbell UL Down Inner Parka at 8.5oz(L).
When backpacking in the Sierra, I wore a l/s Cap 2 zip neck, and had a Houdini along as my wind jacket.
I own a MB thermawrap vest as well, and have used it in combination with the houdini to be comfortable into the 40s (completely subjectively).
However, for 3 more oz, the parka is so much more flexible.
I feel like the Cap 2 + the convective heat loss prevention of the houdini + fleece beanie (1.5oz)will keep me comfortable in any temps the vest would be necessary. At worst, I'd be a bit chilly, and I could always put on the jacket.
Too warm? Unzip. I can't envision a scenario where I'd be too cold not wearing the jacket but too warm wearing it unzipped.
I bought the vest before the jacket, and with access to a time machine, as nice as the vest is, I'd go back and just get the jacket and forgo the vest.