The key to preventing expansion and contraction is to be sure your lauan is running 90 degrees to the direction of your sub-flooring. This way, should one of the 'layers' of flooring want to expand, they kinda 'fight/prevent' each other from doing so. If your lauan is running the SAME direction as the flooring underneath, well, you'll see the results of expansion and contraction.
I hope you're referring to a 'floor leveling' compound when you say 'skim coat'. It doesn't have anything to do with expansion and contraction. The reason it's used is because vinyl is very flexible, and if it weren't used in between the lauan, or where the nails are, you'll see these 'imperfections' in due time after the vinyl has been walked on.