Calcium is what is not needed in a vinyl pool (unless it has a heater). Alkalinity is needed in all pools. It is also called carbonate hardness and is the buffer system of carbonic acid/bicarbonates/carbonates that helps to keep the pH from bouncing all over the place. Use baking soda to increase it. To decrease it see this post
http://www.poolforum.com/pf2/showthread.php?t=191