breathing inside the sleep system
Ouch! Best not to do that, for exactly the reason you discovered -- your breath has a lot of water that will condense in your sleeping bag if you breathe inside it. Before going to a VB, it might be interesting to see how much less water would condense in your sleeping bag if you did not breathe inside it.
I think the problem was that the due point was somewhere between me and the outside of the bivy sack
Under those conditions, with no tent, that is certainly true. The dew point would be in your insulation. You cannot eliminate insensible perspiration (but you can block it with a VB). You can eliminate all else, though, especially moisture from your breath, and you should.
I like climbing into a bivy sack better than setting up a shelter after a long day
Is that still true if, after the long day, it is snowing heavily, blowing strongly, or both? Reasonable people may differ, but riding out a winter storm (and cooking) in a bivy sack does not sound like fun to me. Even tucked in to a comparatively sheltered spot.
worth not having to stop as frequently on the trail to melt snow ... melting snow was our only option.
Best not to have to stop during the day (in the winter). Stopping because you want to is fine, but being forced to stop in nasty conditions is best avoided. Have you tried a controlled eating of snow to get water? Done correctly, it can work just fine (provided you are active enough to be generating the requisite heat). I admit that I have not done so while using poles, and they would make it awkward; I used to do so when I was just using snowshoes and an ice axe.
I don't recall seeing any melt water under the pad in the morning.
What you would see is some ice -- where the snow melted and re-froze. If you see anything other than packed snow under you, you are leaking heat to the snow.