I'm stuck late mid-game - got boatloads of energy/lava/fire/essence, but now I need vitriol or iron to progress. I can't figure out what I'm missing ?

Have you seen a "transmuter" machine or looked through upgrades in essence purifier?

essence purifier requires 20 vitriol to upgrade capacity, otherwise it is fully upgraded

have not seen a "transmuter" machine - should it pop up in the list to buy or in the playing field ?

Looked into local storage, essence purifier says `upgrades: [1,4,2.0]`

Have you figured it out for yourself by this point?
If not: It is made in the Essence Purifier, and is made of a material you've unlocked with the arc furnace.
I would've been more direct, but I couldn't figure out how to spoiler on this site...

Oh, I did not. Thanks for the tip!