https://www.hopkinsmedicine.org/health/wellness-and-prevention/what-does-vaping-do-to-your-lungs

Diacetyl is frequently added to flavored e-liquid to enhance the taste. Inhaling diacetyl causes inflammation and may lead to permanent scarring in the smallest branches of the airways — popcorn lung — which makes breathing difficult.

further down on the page:

Vaping-related lipoid pneumonia is the result of inhaling oily substances found in e-liquid, which sparks an inflammatory response in the lungs.