Pool stores ALL look at calcium levels and if it's not between 200 and 400 they say "You need calcium". No, you don't. Calcium is, as you suspect, for masonry pools, because the concrete, mortar, plaster, and grout all are calcium carbonate and water will leech and weaken them unless there's sufficient calcium in the water. Vinyl? NOT an issue!

Magic additives sold by pool stores never work. Period. I never use them. Watermom never uses them, and most of our support team here never use them. The RARE times any of them might be usable, the owner, PoolDoc (Ben) can determine EXACTLY how to use whatever and what steps to take, but that is very, Very, VERY rare! 99.999% of the time they are a problem and do no good.