At least for me, a VBL is like a Turkish bath until the temp gets down close to freezing. I know some who can't tolerate a VBL at much colder temps. For me, a VBL is not so much for extra warmth as for keeping body moisture out of my sleeping bag and/or puffy clothing insulation on below-freezing nights.

I won't go out without a combination of insulation that will keep me comfy at close to the record cold temps for the area/time of year. Being too lazy to look this up for every trip, I just assume 10*F for high alpine areas. I've needed this for Alpine larch peeping trips in the high northern Washington Cascades in early October--which is as late in the year as I backpack due to my dislike of being stuck long nights in the tent. Also for cold nights in the Rockies, which are much higher altitude.

I suspect that a warm sleeping bag is the most efficient method of insulating. It traps your body heat the most efficiently for its weight. As you said, the gloves (clothing pieces that keep your arms and legs separate) vs. mittens (the sleeping bag).