I think i see what you are trying to accomplish, it looks from the photograph that some of the veneer sits proud of the cement deck, when standing on the deck you will see random pieces of stone that stick out. As the condition exists water will get down into that opening and eventually the freeze and thaw could start poping pieces. If you add azek you will still have a seam that is visible from the top and will collect water and debris. If it where my house i would break up a nice piece of copper, cut a kerf into the cement, and cap the whole detail off. This will take care of water penetration, will conciel the edge variation of the stone, and IF DONE CORRECTLY, will look real nice. G