Well.. Most juice manufacturers try to refrain from using diacetyl from what I know. The thing is, diacetyl is "naturally occuring", meaning that if there's a popcorn/custard flavor, it's being caused (naturally) by diacetyl, one way or another. At least, that's what I've been taught. Nobody puts diacetyl in their juices.
That said, there are other chemicals and stuff that makers DO put in their juices, KNOWINGLY. One of the vape shops I frequent (they brew all their juices in-house) have, on display, all the chemicals that are used in juices, and if you want, the owner will let you take a little bit of the powdered chemicals and taste them, because the thing is, these chemicals are used in food, and in food, they're totally fine for you. But when vaporized/aerosolized, they can build up in the lungs and gunk up your "cilia" (which are like the toothbrushes of your lungs, catching dust and stuff), and some of them have been linked to kidney stones (very painful, do not want). The chemicals have [mouth] tastes eerily similar to some juices that I've had, and on top of that, these chemicals tend to vape the same way they taste.
As a joke/PSA of sorts, the shop owner mixed up some e-liquid with the various chemicals, and has them behind the counter with skulls/crossbones on them. If you ask to try them, he'll let you, and the VERY scary part is that they taste REALLY good. One thing we do know though, a little bit of any of these chemicals, whether it's diacetyl, or the one that people have been using to make that classic "french vanilla" flavor (I think it's just called vanilla sulfate or something), isn't gonna kill you. Your lungs recover amazingly quick, and even if you vaped 30mL of Diacetyl Berry Crunch (someone should actually make that a flavor), as long as it's not your ONLY eliquid and you're not continuously vaping it over the course of a year or two, it's not going to have any notable adverse effects.
A good way to tell if your juice has some kind of gnarly chemical flavoring in it is to taste the juice. Not by vaporizing, just put a drop or so (don't do this with 12+ mg nic juice PLEASE) on your hand and lick it off. Does the juice taste not-so-great? Does it have a sort of "oily" feel and not taste like what it vapes like? Well.. Good! That's actually how juice should taste. Sort of bitter and weird mouth-feel. If your butter/custard juice tastes exactly like how it vapes, it's suspect. I don't wanna name names, but I feel it is worth mentioning: Epicus Nebula. That juice POSITIVELY has SOMETHING not good in it, and it's really irresponsible because this local juice brewer is gaining popularity very quickly, and the juice is good, so like without a warning that, "This juice has xxx in it, vape at your own risk" the maker is basically poisoning his customers. If he knows that, he should be put in jail, but if he doesn't, he isn't responsible enough to brew juice.
It's just worrying, a lot of these people brewing juices just don't realize that they need to use SPECIFIC vape-friendly flavorings. For this reason, I (and the vape shop owner I've been mentioning) support the FDA, and we really WANT juices to be regulated. At the very least, the FDA would begin screening out/making people aware of chemicals they should not be vaporizing.
