Further to Carl's excellent post and other threads on this topic - If you have a heater, warranty requirements may state you have to have a certain level of calcium, and that's about the only reason I've seen given for bothering about calcium in a vinyl pool. Though "Liner wrinkling" is an interesting new scare tactic for me.

If you do want to raise caclium you can do it with the granulated chlorine with calcium or find a pool place that sells a big bucket labeled "calcium" and not marketed with a fancy name.

I don't argue with the pool store folks, I just nod my head and say "ok, I've got that at home..."