How quickly your pH rises (or not), depends on a couple things, most notably, the pH of your fill water and then the TA of your water.
The TA buffers how quickly the pH rises. Generally speaking, lower TA numbers in the 50-60 range, although outside of TFP recommendations, work very well at pH buffering in Vinyl pools where worries of CSI isn't a big factor.
It wouldn't suprise me that if you tested your TA, you would find this might be the case.
I would be surprised if your TA is above 70, and have never had to adjust the pH.
I have a vinyl pool IG pool, My TA stays between 50 and 60, and my pH has been steady at about 7.4 - 7.5 for 3 years now. I also have a SWG.