The bottom line is it really doesn't matter, so do what you enjoy. VIT is never a bad idea, though as a Scholar you have a dedicated healbot fairy that drastically improves your lifespan. As such I never saw VIT as critical beyond what I could find on gear. Piety seems like a neat idea because it would help both scholar and summoner, but after playing I've discovered that Scholars really don't need any help supporting their mana pools & are fantastic long term healers.

In my (more limited) time as a summoner I felt myself often dipping into an MP deficit. So piety could perhaps be quite beneficial as a summoner considering Aetherflow restores a % of your max MP rather than a hard number.