Your bottom vent just needs a bead of simple silicone sealant. It is cheap and can be purchased at MANY stores. I do recommend that you go with "clear" though. Now if you really want to do it the RIGHT way, then take out the fire ring and the firebox. After cleaning you will see the nuts on the back side of the 4 bolts holding that vent in place. Remove them and the vent, clean all the old sealant and apply a fresh bead all the way around and then re-install the vent. Done properly that way then that should solve that problem for the rest of your life!