This was a second floor repair. Nevertheless we always use #15 felt paper as a vapor barrier between hardwood/plywood. In those pictures I had already cut out the old felt to expose the mold.
Subfloors over concrete are supposed to have 6mil plastic between the concrete and 1st layer of plywood. It is also supposed to wrap up the walls until installation is complete, at which time it's cut flush with the finished floor. Houses built on crawl spaces are SUPPOSED to have black 6mil plastic on top of the ground throughout the entire crawl space.
Plastic is not a good choice between hardwood and plywood. In the event of a surface/interior leak, the moisture has nowhere to go. It will sit, mold, and not dry out. Felt paper seals around protruding nails and cleats well and will allow transferrance of the moisture. In this case somehow the plywood still got some mold. Your cabinets are awesome! Well done.