I Haven't seen any juice or flavoring for years that directly use diacetyl. Acetoin, Acetyl Proprionyl taste similar, butter notes, and can break down and form some diacetyl (4-10% of the original chemical amount), along with some fruit flavors. There was a claim here, which I haven't seen repeated, that butyric acid increases this breakdown. Butyric acid is used in yogurt and cheesy flavors with another butter notes, and is the most common substitute butter note. It is also the primary flavoring in vomit and an irritant. A person who has tried all of them claimed here that of the 4, diacetyl is the least irritating. Flavourart claims what they do results in no butyric in the final flavor, but I'm unimpressed with the flavors I've tried, for example vanilla custard is more like just vanilla plus a little bit of odd flavor.
When I use creamy flavors, I stick with low amounts of flavorings using Acetoin or Acetyl Proprionyl, although some have become difficult to acquire. I'll go to unflavored before I vape that TFA DX stuff.
Large amounts of diacetyl are released from fresh ground coffee.