I did several weeks of testing this summer with all kinds of space blanket arrangements and a silnylon undercover and the OCF foam pad made by Hennessy. With temps to the low 50's F and humidity well over 90%, I couldn't get more than a fine mist of condensation below the hammock.
I wouldn't doubt that more condensation could be generated in just the right conditions, but I would be really surprised to see it happen often or amount to much.
Having a space blanket just below your hammock does have some vapor barrier effect, but not like a conventional vapor barrier sack used in very colder weather to protect insulation from accumulating body moisture. First of all, it is an open system and not at all sealed tight, so there is plenty of opportunity for air movement, like every time you shift in the hammock. And warm air goes up, not down, again leaking around your body and sleeping bag or quilt. Your movement pumps some air in and out of the topside insulation too.
You can feel the radiant effect of the space blanket when you climb in, no doubt on that for me. I think that most of the real advantage of the space blanket layered with the SS under cover and pad is just cutting down on air circulation and keeping any cold air away from the hammock bottom and the user. Wind is pretty much blocked from the hammock by the "sandwich" too.
An under cover with a partial UQ is nice too. It blocks wind and forms an additional layer of dead air. The ends of a partial UQ are vulnerable areas for heat leaks and a cover helps. A little attention is needed so the cover doesn't compress the quilt and lose loft. With either a quilt or the SS OCF pad, the cover should be close but not quite touching.