A water, moisture/vapor, air barrier is better to be in place, such as some kind of flashing, elastomeric products, or similar.

I put up vapor barrier, fiberglass insulation and drywalled.

The cement floor underneath the carpet apparently has no vapor barrier.

Got into a dispute with the developer as the foundation was missing a vapor barrier.

Be it glue down or vapor barrier flooring if you have areas that need leveled with float do it! 

Sure a lot of start ups will vaporize (by design, most fail), and there will be a few spectacular unicorn crashes (too much cheap money went there).

the house is a ranch on about a 2' dirt crawl with a vapor barrier (yeah, you REALLY gotta crawl).

Drawback is the application and vapor.

If cracked, combustion vapors will get blown into home circulating air, like if a radiator has a leak the fluid will spray out slowly, right. 

Smoking is considered cigarettes, cigars, pipes, hookahs, water pipes, vapes, e-cigarettes, and any other like kind instrument that produces smoke or vapor, regardless of whether tobacco or other consumables are used.