I really don't know, but many chemicals exposed to new environments (i.e. water) will leach out especially initially before reaching more of an equilibrium. We know, of course, that plaster works that way, but that's a specific chemical process of curing. With vinyl, I'm sure it's not so dramatic, but something like that could be happening.

I would say that if many people with new vinyl pools find initial pH dropping or not rising as quickly and then later find it stable or rising faster, then you very well could be right. Otherwise, this is just one more mystery without an explanation.

Richard