The past couple of years have been a bit of an anomaly here in the South as Winters have reverted back to what they were thirty years ago. While temps would occasionally dip down well below freezing, most of the time temps would be at or near freezing. Air below freezing tends to be quite dry and it is these conditions that insensible perspiration is at a higher rate as our bodies attempt to keep our skin moist. Above freezing temps tend to make for wet UQ's and TQ's in the mountains around here as the dew points stay near air temperature resulting in condensation on the outside layers of insulation. (That's where weather shields come in handiest).
Either way, wet insulation is not your friend. Most of us don't go out for more than a couple of nights at a time, and the cumulative consequences aren't really that bad in those situations. Multi-day trips in sub freezing temps are where vapor barriers offset their weight in terms of benefits.
Personally, I just think it's kind of cool to wake up to a bone dry TQ the morning after using VB clothing.
As far as IX being a VB, I've been using a small piece of IX folded over the head end of a TQ (in lieu of Shug's bib). It appears to be working for that purpose so far.