Many are interlocking and float over a vapor and moisture barrier.

Removed and redid the ceiling drywall and vapor barrier.

Don't believe me, check this out from energy.gov http://energy.gov/energysaver/articles/moisture-controlA poly vapor barrier can help in these areas as well, again as long as you tightly seal the area - around doors, windows, outlets, etc.

When I last had to replace a line set, the tech said that if the line has been open to the elements for a prolonged period of time, particularly during high humidity, that there's a fair chance that enough water vapor and whatnot has gotten into the condensor that there will be problems down the road.

If I am doing tile I just use the mortar and tile for the vapor barrieor.

Is there a proper vapor barrier or radon mitigation system installed if needed.

I was planning on converting it to a SDIRA and buy a rental once I see whether or not I stink at picking rentals and being a landlord - I don't want to vaporize my IRA.

You may have a condensation problem if there was a vapor barrier between the plaster and sheetrock.

If not, I'd fill in the missing pieces and have the vapor barrier down, which is the practice in my area.

How about putting down some vapor barrier plastic?