[rusty baker]

Ok. To do it right, you need 1/2" plywood over the planks. And then for a smooth surface for the vinyl, you need 1/4" underlayment grade plywood stapled with narrow gauge staples, every 4" to 6" in the field and every 2" to 4" on the edges. The ends needs to be staggered and I always filled the seams with cementious based fill, although some installers don't. Might as well do it by the book, if you are going to do it. I am Armstrong vinyl certified (went to their vinyl/lino school) and have been installing since 1973. And you do need a small gap between the plywood and the wall. All wood expands and contracts.