Yes, a vapor barrier liner does add considerable warmth under cold conditions. I normally use a light down bag that's comfortable down to about freezing. By adding a very light vapor barrier liner to it I've been comfortable down to about 0F. It's not due to extra insulation but by reducing the heat lost through evaporation of perspiration during the night. It also has a side benefit of reducing water loss so I don't wake up thirsty in the morning.

I first discovered the benefit by accident on an unexpectedly cold night when I was still shivering in my usual bag. I remembered that I also had one of the emergency mylar blankets along so I wrapped myself in that inside the sleeping bag and it made a surprisingly large increase in my comfort that night. Later read about vapor barrier liners and eventually bought a nylon one (the mylar worked fine but is noisy whenever I moved around a bit).

I don't use any liner in above freezing conditions.