I have nothing scientific to add. However, I have on a number of occasions woke up in the night uncomfortably cold and slipped on my sil-nylon or plastic poncho and slept comfortably warm for the remainder of the night.
On extra cool nights, I have donned my poncho upon going to bed...and about 40% of the time, I put left over bulk-food plastic bags on my feet held on w/ rubber bands around my ankles...before going to bed.
With the exception of one freezing night that I recall, all of this has been done in temps above freezing (mid-high 30's...perhaps low 40's.
On the nights I woke up cold and donned my poncho, I start feeling more comfortable almost immediately.
I should also note that I have tried these type of VBL's over my bare skin and over light fleece. To me, the all-around comfort level is far greater when the VBL is over fleece. When I remove the VBL upon arising, the fleece is always slightly damp but dries within minutes of being exposed to the air.
I have had such outstanding results with the above that some sort of VBL is an essential part of my kit now...and I rarely encounter below freezing temps.