I don't know if your asking Steve or me. Heres my thoughts. Yes ,at least 2. As Steve wrote,keep in mind ,will the doors open and close with 2 layers off and adding underlayment and new vinyl. Leveling compound is ,in my opinion,is very difficult to use. If you don't have it perfectly smooth, you'll see and feel it. This is why I recommend underlayment. Ask what your lumberyard carries for this project. I don't know what Pro-Board is but do not use actual plywood. The underlayment will give you a much better surface to work with. Most people tell you to fill the joints,but i don't. My floors never show any joints,sheet or squares. Another thing to remember, YOU MUST set the nails below the surface. Just use your hammer, but don't cause a large dimple or that will show. If you think you can get a smooth surface with compound , by all means , go that direction. I'm sure it will go OK either way. Have fun enjoy doing it yourself. Ron J.