Hi,
First thing is DON'T PANIC. Your numbers aren't bad. 7.8 is still in the acceptable range and 90 for your TA certainly is.

Here's what I would do: Lower TA to about 60. Yeah, 60, and see if your pH holds at a lower level. My pool was at 60 and didn't MOVE from 7.6 for 6 or 7 weeks. You can easily raise your TA a pound of baking soda at a time if it bounces around too much. You have a vinyl pool. TA isn't that critical unless it's too high (like around 200).

To lower, just add acid till your pH is between 7.0 and 7.2. Aerate the water (fountain, kids splashing, pointing the return to the surface) and pH will rise without T/A increasing. When it hits your target, say 7.5 or 7.6, stop aerating and check T/A. If it's between 60 and 70, you are done. Now wait to see if pH holds. If not lower TA again via the same method. We are seeing more and more pools doing well with lower TA levels, including my own.

Carl