What is the proper waterproof bathroom underlayment for vinyl tile flooring?
Concrete board would be your​ best bet as long as it is smooth if not you will have to use floor leveler and I would suggest using a mesh tape on the seams with floor leveler or other type of compound so the tile does not crack. There is many types of waterproofing you can just roll on with a paint roller and paint brush. One other thing to consider is using a glue both waterproof and keep the tile in place.