Certain flavorings contain a diketone chemical. They were mostly used in butter or cream flavors.

Orignal vape juice contained Nicotine, Vegetable Glycerin, and Propylene Glycol. Then a flavoring was added to make it taste.

The flavorings are used in baking or even making popcorn. “Safe” to eat but not to inhale.

People that worked in movie theaters would develop “popcorn lung” from breathing in the butter flavoring all day. That’s where the term comes from.

Any reputable vape juice company would not use these flavorings today.