I agree with you about the water guides being a boilerplate so not very helpful in figuring out whether saturation with calcium carbonate is really needed.

I got some more info I posted here at TFP where the most interesting information is that 1) calcium carbonate levels below 7% in vinyl do not significantly affect its physical properties and 2) that at such low levels even acidic conditions didn't result in a loss of flexibility and relatively little loss of weight.

So to me this means that low levels of calcium carbonate in vinyl pool liners probably wouldn't require saturation of calcium carbonate in the pool water. Though high levels of calcium carbonate in vinyl might require such saturation, such high levels are stiffer vinyl that is more susceptible to acidic conditions and general chemical attack. So hopefully, most vinyl pool liners don't contain such high levels of "filler" calcium carbonate. It's too bad there isn't an easy way to tell, though if one had a sample of the vinyl then I suppose one could see the weight loss when exposed to very acidic conditions since that presumably roughly measures the calcium carbonate content. Unfortunately, most people won't have a piece of spare vinyl from their pools let alone accurate scales and even then this test may not be definitive.