I would recommend a backsplash. It will hide any imperfections in the fit between the vanity and the wall and just adds a finished look. You can use a 1x5 or 1x6 piece of solid Oak or other nice wood to will match your decor. Route an ogee or other nice edge on the top and sides. A piece of 1/4" quarter round attached with caulking will finish the edge across the bottom.