For what it's worth, I have never lived in a house that has a true vapor barrier above ground and don't know of any houses that have them. Every house that I have seen around here has either tarpaper or tyvek or a similar product over the wall boards and under the siding. A true vapor barrier would be plastic sheeting under the interior drywall. Some people say that the have vapor barriers and advocate putting them in the ceiling under the attic also. I don't and would not. You always have moisture inside the house and want it out. This comes from showers, washing clothes and dishes and similar activities. If you have a true vapor barrier, it seems to me that you could have condensation of interior moisture on the vapor barrier on a cold day and I don't want condensation occurring inside my walls. What I do want is a wind and water infiltration barrier under the siding which is the job of the tarpaper or Tyvek.
I don't think that mold, mildew and moss growing on the outside of a house has much to do with any of this. Air motion--wind--and sunlight are great preventitives of this. Why do I have a nice patch of moss growing on the tree bark? I don't think that it is from interior moisture from the tree. It is from the rain and little sun under the tree.