I think it was Backpacker Magazine that evaluated reflective liners back in the day when "Texolite," a mesh-protected, perforated foil, and some other reflective barriers were being tried by several bag manufacturers.
The conclusion was that the reflective barriers lowered the degree ratings of the bags when placed just inside the inner shell, but not when placed inside the outer shell. Sorry I don't have the article for you. At that time, Backpacker was doing a pretty good job with issues like this. As I recall, however, the reflective liners were all perforated, and the bags were vapor permeable, not VBL. Don't know if VBL would make a difference.
At that time, I bought a 'Yaksack' from Yakworks, that used Thinsulate with a Texolite layer just inside the inner shell. It was amazingly warm for its 2 lb. weight. I recall using it at the old Tamarack shelter in winter on Killington Mtn in VT and not being cold at all. Then I washed it a few times, and it became no warmer than other synthetic bags of the same weight. It couldn't have been a loss of loft, because the Thinsulate didn't depend on loft. The shell was laminated to the insulation, and the bond was lost; but I don't see how that would affect the insulative value. So I stopped using it. It's always been a mystery.