I plan on using a Bivy for winter camping, in a tent only. First for a little extra warmth, and second for keeping any moisture off my down sleeping bag ( snow from other gear and frost from the inner side of the tent in the morning ) also a little extra protection for the sleeping bag itself. Will this Bivy keep my sleeping bag dry on the inside from the heat and moisture from my body. It will always be open and I will not be breathing directly into the Bivy it's self.

Any help is greatly appreciated

I have this bivy and too many different bag combo's - if your looking for extra protection, if you can spend your $ on a bag with moisture resistance or treat your current bag. For a vapor / moisture barrier, an extra pound is lots to carry speaking from experience. If your sleeping in a tent and I assume you have a properly rated bag yet are concerned with MOISTURE during winter outings, rethink how you are setting up your tent to get MORE ventilation / airflow to avoid moisture issues. That said, all other considerations aside, yes, this bivy will help keep your bag dry but with significant weight and volume penalties that gotten around with a little extra care and buying a better winter bag. **If you suffer from excess frost/moisture and if your looking at this as a solution to avoid needing to increase the amount of ventilation in your tent, it is going to deliver lackluster results at best.