How should I prepare a plywood subfloor for floating vinyl plank flooring?
I'd question how waterproof you need this to be. Will there routinely be water sitting on the floor? If not, you may be over-engineering things.

That said, off the top of my head, one option for waterproofing the subfloor would be to paint on Redgard: 



It's a 'paint on' waterproofing membrane that you'd typically use behind tiles on cement board and the like.