WPB layers are designed NOT to be a VB. They are designed to block water while letting vapor pass. They get wet on the inside when the rate of vapor generation exceeds the ability of the fabric to pass it. When you're in your hammock, you hopefully will not be generating a lot of sensible perspiration. The rate of insensible perspiration a body generates should be able to pass through a WPB fabric.
As BillyBob and I mentioned, hikers have been using WPB layers to dry clothing with the outward transport of vapor. This would seem to confirm that WPB is not VB. It's more like "vapor slower".