Popcorn lung came from inhaling vape juices that had Diacetyl, or Vitamin E acetate.
The nasty cases were Vitamin E acetate, not diacetyl. Diacetyl is butter aroma, same stuff as in actual butter, probably not dangerous at all at sane concentrations, certainly at the amount sensible for vape juice. Cigarette smoke contains more diacetyl than the highest ever measured concentrations in vapes and somehow noone is arguing that cigarettes are causing popcorn lung – they do nasty shit, but not that particular kind of particularly nasty. The original popcorn lung cases were workers in a factory producing popcorn and handling the stuff pretty much pure, though TBH I’m not entirely convinced it was actually diacetyl and not something else.