The sprayfoam is mold proof and vapor impermeable, thus sealing it off. 

So if the homeowner wants to replace the particle board with OSB or plywood they have to pay the difference which in the overall scheme of things is quite negligible and well worth it.Belly boards aka vapor or varmint barrier is always a problem and needs to be checked regularly as plumbers and other tradesmen will tear it from limb to limb and not repair or replace it correctly if at all.

There are defenses to liability for property owners whose property is impacted by contaminated groundwater from an off-site source so long as you exercise what is known as "reasonable care". (2) since you will be using this for self-storage, this will be considered a commercial, not residential use, and many states have less stringent standards for commercial properties since people are not occupying the property 24/7. (3) If the levels in the groundwater would pose a risk of vapor intrusion (vapors migrating up from groundwater), you can install a passive sub-slab depressurization system (ssds) that acts like a radon system-capturing the vapors before they can get into the structures.(4) if soil is contaminated and you plan to pave the surface, this will generally be sufficient remedy if the soil contamination is at depth.

Got an expensive termite & crawl space repair needing to be done before closing required by the underwirter, totaling about 8K, including termite treatment, organic growth treatment, digging out dirt for enough crawlspace height clearance, stiff legs insulation, vapor barrier, 6 crawl space vents (2 powered & 4 regular), and 5 vent wells. 

Once you get into framing, there's a lot of things to know about vapor barriers, ground contact lumber, mold-resistant materials, etc.In my opinion, this isn't a DIY type of project unless you're a licensed contractor.

What mil thickness vapor barrier if any would I need.

be sure to remove the vapor barrier 

There is no vapor barrier on the concrete floor, they just laid down strand board then carpeted right over.

The heat turns the water to vapor and the ventilation allows the vapor to escape from the clothes and drum.

It does not give off flammable vapors that will ignite from a spark.