My opinion is that there isn't an additive temperature improvement by adding a vapor barrier to breathable insulation. I think it is a multiplier that you apply. There are a lot things going on to get the 20F per inch of breathable down insulation guideline that we often use. There is certainly the R-value of the insulation (or clo-value or tog-value) of the insulation itself but that number is applied to the metabolic rate (or something, it has been a while since I looked at all that so that might not be the correct term) of the individual. It is that metabolic rate that you are modifying with a vapor barrier placed between you and the breathable insulation. So that 20F per inch might become something like 25F per inch for breathable down insulation with a vapor barrier in place.
So with that, if you add just a vapor barrier to a hammock you might only get a degree or so of improvement. If you add it with 1 inch of breathable down underneath you would get a 5 degree improvement or 10 degrees with 2 inches, etc.
But this is just the way I have it figured based on what I have seen, experienced, and surmised. I don't have references to back it up. I haven't seen it characterized this way, at least I don't recall seeing it.