I was intrested in this as well, a quick wiki search shows that diacetyl is naturaly found in dairy, and the compound used to flavor fake butter, and is the primary additive in microwave buttered popcorn, and identifide as the cause for the degenerative and non-curable disease found among those involved in the manufacturing of the popcorn which gave it the nickname popcorn lung. As a side note, diacetyl is also found as a byproduct of fermentation in wine, and beer. It seems that it dosn't cause problems when ingested, but not something you want to
vape for sure... anyway... it seems that the use of natural flavors doesn't preclude its presence, but most likely not in any amount that would be the equivalent to an artifcially flavored butter popcorn vape, by someone who doesn't know what they are doing.