We have an old cabin that has an old wooden floor. Several of the boards are bad and there is no vapor protection underneath the floor. The cain is on pier and post. We would like to put in a new sub floor out of pylwood and then lay a new floor on top. Where should I put the vapor protection at and what should I use. What should the thickness for the plywood be? Any other suggestions?

How high off the ground is the floor now? What area of the country? What are the posts/beams/joists made of? What size joists?

Need a little more info. Floor joist spacing and size will determine thickness of plywood. How big is the cabin? Since you have such an open foundation, many people with construction like yours will install open face insulation and them 6 mil poly across the floor joists. How is this cabin heated and cooled?