Well, Ms. Richards, do you hate the existing floors or are you just hoping to improve air quality and energy efficiency? Do you have old-growth heart pine that SHOULD stay or something different? We need to know that. Are we trying to kill two birds with one stone, replace floor AND improve air quality/energy efficiency?
Now, all the old flooring guys and all the manufacturers will tell you that you have to have a vapor barrier installed on the subfloor before you install your finish. That's true BECAUSE the crawl space/basements are never lined/encapsulated and dehumidified. Buckets of moisture are migrating up out of the soil into your investment(lay a 2' x 2' piece of clear plastic on your lawn in the middle of summer and watch how quickly water droplets form on the underside). But, maybe you want to keep what you've got ---- the floors are fine! Seal basement, line, up the walls, seal the foundation vents (definitely) and dehumidify(HEPA filter will remove micro contaminants) .
If, IF!, you are replacing the finish floors, that's a completely different conversation, one with the entire building health in mind especially with regard to how you treat the subfloor and regarding the addition of a second vapor barrier.
I hope this helps.