My goal for overwintering VFTs and Sarrs is to limit the number of freeze-thaw cycles and to protect plants from dessication in wind or sun. There must be a temperature that a VFT can't drop below for long and survive, but it must be well below freezing and it doesn't seem to be an issue here in CT.
I'm in Zone 6, our coldest winter temperatures usually run 0 to -10 F and my VFTs do fine being frozen solid for much of the winter. But I bury their pots in loose soil and cover them with a loose pile of oak leaves. That provides protection from the most extreme winter temperatures (cold and warm) and allows some, but not too much light and ventilation. Better still, I believe they freeze once and thaw once, avoiding a lot of root and cell damage.
By the way, although snow is a good insulator, it can be dangerous for plants underneath when it melts on top and the water percolating down re-freezes below. That suffocated some of my plants several years ago, so now I erect a little lean-to that keeps off most of the snow.