if these are peel and stick i doubt that they will stick very well to the hardibacker. 1/4' luan underlayment would give a smoother surface. that is what is usually installed under vinyl floors. It's also cheaper then the hardibacker is. does he want to raise his floor the additional 1/2 inch? they recommend that you prime the floor with a primer made for peel and stick tiles. the odd thing was i went looking for reviews for this product and couldnt find any bad ones. in my experience it doesnt matter what the product is there are a;ways people that have a bad experience with it. I just thought it odd. I have never heard of hardibacker being called for under vinyl flooring.