Buying part of brother's house

[Brad Smith]

Bought a new house in December.  It's a pier and beam foundation but there are some soft spots.  The flooring people say that one of them needs to be repaired.  I made an initial repair but it was not very extensive.  I've since gone back in and removed all the rotten material.  My understanding of the way these are constructed is that the subfloor sits on the floor joists but this one seems to have two layers of subfloor.  There are 2x8 (going from memory) with plywood on top of that.   The plywood is what is rotten.  There is a moisture barrier between the two layers.

Question #1 - Is it typical for there to be two layers of the subfloor or am I describing it wrong?

Question #2 - can I use a plastic bag for my moisture barrier when I replace the plywood or is there something more appropriate?  A spray on product would be nice.

Thanks