You will have to be in subzero temps to make a vapor barrier around your hammock even begin to sound like a good idea.

Look at hammockforums for winter camping solutions. Or look up Shug on youtube, dude camps out in subzero temps in hammocks all the time with your basic underquilt and pad setup. Or Turk - he pulks out with peapods, underquilts, hammock hut and a nice little stove, and there are videos about that too.

Jacks R Better and Speer have four season underquilt/peapod solutions.