For maximum effectiveness, lower the pH to 7.2 before adding such a large amount of chlorine and for both the initial acid to lower the pH and for the chlorine addition, be sure to add it very, very slowly. I would add both over the return flow in the deep end and then brush the sides and bottom in the area after addition just to make sure it's thoroughly mixed. For sure do not add the acid to the skimmer, but some people do add the chlorine that way (I'm not as certain it's as good as the return flow into the main body of water). I'm just concerned for your vinyl liner with concentrated chemical addition (I'm not concerned with the high diffuse FC level).