The main issue is that vendors (of e-liquid) take [flavor] manufacturers at their word with zero test results to prove otherwise. For those that use FlavourArt and The Flavor Apprentice, this really shouldn't be an issue as they both provide details. TFA even provides MSDS's and CAS #'s and from what Linda has repeatedly told me, their manufacturer isn't even allowed to have Diacetyl in their factory due to insurance restrictions, so if Diacetyl is present, the resulting trace amounts have catalyzed from Acetoin, even though she has also stated that it's unlikely once a flavor is mixed.
When it comes to flavors from LorAnn, Capella, FlavorWest, Fairies Finest and other select flavor providers / vendors, you're pretty much SOL as they either make a blanket statement or they won't say anything / provide proof. With these vendors, you either have to test or make a blanket statement stating that the e-liquids sold may contain X, Y, Z (better than saying nothing, or making a broad claim that you can't backup).
Hangsen & Inawera are iffy. They claim Diacetyl free (via e-mail - I've been in touch with them both), though there are flavors from both that I feel contain Diacetyl or, at the very least, Acetoin and/or Acetyl Propionyl. I did receive a report on Hangsen Vanilla, which is one that I was the most iffy on based on taste, but the report which listed the chemicals didn't indicate the presence of any of the 3; not even butyric acid which is one of the newer chemicals to surface that's being used in place of D-A-AP (which, IMO, is equally as bad if you're going to compare it - just take a look at the description of the chemical).
FlavorWest, IMO, is the biggest offender and many companies use them from what has been said. Their butterscotch flavor was one of the flavors tested by a Canadian vendor only to find that Diacetyl-Free actually meant more Diacetyl than most other flavors on the market that still contain it, which was a bit shocking.
