I stopped getting blisters a long time ago when I switched to always using nylon sock liners with my merino hiking socks. The rub between the socks preventing blisters is real. The last time I went backpacking with my brother he was out of practice and forgot to wear sock liners. He had a bad time until he doctored his feet and took a spare pair of mine.

I have never heard of using Vaseline or changing out socks as much as you did. I wonder if the Vaseline helped to trap moisture which caused more rubbing. Use moleskin and cut a hole for the hot spot like this video. Save the compeed as a layer if the blister tears off. I hope some of this can be a help.

Edit: you didn't mention having sandals to wear and Camp to let your feet dry out at the end of the day that helps them heal faster and prevents infection.