If only someone made a...
Cuben Fiber vapor barrier suit. I'd like one with a hooded jacket, but no zip, something like the absolute minimalist design of the BrawnyGear Rain Suit. And Cuben Fiber vapor barrier mitts and socks as well.
And as a bivy sack doesn't need to be breathable if you're using it in conjunction with vapor barrier clothing, I'd like one made completely of Cuben Fiber too. It would weigh less than a breathable bivy sack, and would also be completely windproof, thereby reducing heat loss by convection.