From my own experience--silk liners add little or nothing to the warmth of the bag. While using it I would be so tangled up by morning that it would take me several minutes to get out of the sleeping bag.

For conditions below freezing, a vapor barrier liner inside your bag is good. It keeps the moisture from your body ("insensible perspiration") from going into the bag and freezing when it hits the outside shell, which can compromise your insulation. If your bivy is not highly breathable, you're going to have condensation inside it, which will also compromise your insulation. The vapor barrier liner will help with that, too. However, you don't want to be using the vapor barrier in above-freezing temps, because it becomes a sauna!