The "correct" PH depends on other factors like TA and CH which is why I suggested the calculator. However, having a negative CSI does not guarantee a scale free cell, only reduces the likelihood. You could try lowering the PH a little and see if that helps (i.e. keep CSI between -1 and -0.5) which should not be a problem since you have a vinyl pool. Also, it would be a good idea to make sure you are actually getting scale and not something else. Scale is indicated by fizzing when immersed in a acid solution.