Hmm, I perhaps could have seen this coming.
As soon as I added the acid to the VSC8512, with the large copper plane that I had sanded the soldermask off of to image, it started reacting aggressively even at room temperature and throwing off more NO2 than I was comfortable with.
I ended up pouring the acid off to a separate beaker to let things calm down a bit. Will give it a few minutes then see how much copper is left... I might want to do a pre-etch with a dilute nitric solution or something so i can remove the remainder of it without it getting too energetic.