Tim Watts wrote:

If moisture IS present, you install a "vapor barrier' - essentially
plastic film - before installing the laminate underlayment then the
actual flooring.

If there is NO moisture found after the test, you can dispense with
the vapor barrier, although it certainly won't hurt to have one
anyway.

Agree - the cost of a sheet of plastic is peanuts in the grand scheme.

I don't think it's EXACTLY peanuts. Vapor barrier material may be as low as
five cents/sq ft (impregnated paper) up to, if it includes cushioning
underlayment, up to $0.90/sq ft.

Note there is NO difference between a vinyl vapor barrier and 6 mil
contractor trash bags. With the latter, you'll do a bit more cutting and
taping. That's all.

Or you can use roofing felt, which is probably the easiest to install and
pretty cheap ($0.11/sq ft from HD).