Welcome to the site.
Yes, you can use a 6mil plastic for the vapor barrier. Not sure about putting it down and installing over it. I can see where they would cut it up for various installs, plumbing, piers, etc.
It is easy enough to do once the home is set into place and before the skirting is installed. Make sure to have each piece over lap the first by 6 inches, we tapped ours. Also it is a good idea to go up the piers the same amount and secure into place.
Look forward to helping you in the future.
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