I would like to make a technical point that is often "incorrect" in my opinion - vinyl liner pools do need a certain level of calcium in the water for two very different reasons.

#1 - calcium acts as an anti-foaming agent in water. A concentration of 120-200ppm is all that is needed to arrest foam formation.

#2 - unfortunately there's not a lot of good technical literature on this subject but many polymers, vinyl included, use cheap calcium carbonate additions (along with plasticizers, release agents, etc) as "additives" to the polymer matrix. Calcium carbonate modifies the tear strength and hardness of vinyl. So, in a residential pool, water with no calcium and low pH can strip calcium from the vinyl liner material if calcium carbonate is used as a filler material. Unfortunately, there's no way to know how much calcium is used but I have heard figures quoted as high as 7-10% by weight. Only the actual liner manufacturer would know the details of the vinyl starting materials that they use.

Just a minor point to make. Calcium is going to get into your pool water one way or another, so unless you go out of your way to reduce it and keep your pH unnaturally low, CH (calcium hardness) is not a big deal for vinyl pools.