I just went through something very similar. Opened pool to low PH (6.8 or lower), TA was OK, vinyl pool. I did BOTH, aerated and added borax. Why? My PH was low enough that I was worried that the vinyl liner might wrinkle as the pool warmed. So I started aerating (which did bring the PH up, but somewhat slowly) and added a large box of borax. After a few days, everything was fine.

The borax was an insurance policy, it made me feel better to get the PH up fairly quickly.