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?

The term “smoking” means inhaling, exhaling, breathing, or carrying any lighted cigar, cigarette, or other tobacco product or similar lighted product (Including but not limited to e-cigs and vaporizers) in any manner or in any form.

Also this building was built without any kind of a vapor barrier,  just T-111 siding right over the studs. 

Maybe 6 hours labor for crawl space + add in a vapor barrier.   

This is some prefab construction built circa 1960, if I'm not mistaken First, I would go with a plastic six-mil vapor barrier.