I'm just throwing this out there, but if the owner replied "nope, no diacetyl here", would you believe them?
I think this goes beyond what is and isn't in a juice. This bottle of mystery Chinese RY4 I'm vaping could be made with reconstituted rabbit pewp with a splash of nicotine from steeped cigarette butts and a melted down caramel chocolate bar in it for flavour. I have no idea, and neither does anyone else. Because there is no control over juice, or more specifically what exactly is in the juice (or the flavourings), we put faith in juice makers that they aren't selling juice containing nasties. Its a risk we all decided to take when starting vaping.
I have my favourite vendors, and I trust that these vendors aren't out to harm me by knowingly selling a product that contains deadly (to the best of our knowledge) chemicals - and I'm ok with that. That's why I continue to buy from these vendors. You'll need to email vendors, ask about their juices, then make your own decision about who you can and can't trust.