If you use a vapor barrier on the top of the tent you will prevent it from breathing. This may cause considerable moisture (condensation)inside at night. I've used the poly paint covers that you get at Lowes etc. to pitch my tent on. I don't know if a tarp will give you any more insulation. A tarp above the tent should help prevent heat loss. Using a candle lantern inside can bring the temperature up. Rigging a tarp as a wind barrier also helps.