Most existing deck surfaces (PVC membrane or liquid applied finishes) are applied to 5/8 inch or 3/4 inch plywood and therefore are not suitable for a tile overlay without some stiffening to reduce the deflection. The vinyl could be removed (possibly leaving the existing vinyl up the wall behind the siding and building paper), the floor stiffened with the appropriate thickness of cement board and the Tiledek installed over top. There is another system that can be used to stiffen the existing structure. Schluter's Ditra mat can be installed over top of Tiledek (with thinset) and then the tile can be installed over top of Ditra. The Ditra mat stiffens the floor system to an appropriate level to provide for residential use of tile. We have tested this system with the Robinson Floor test and achieved a "residential" result.