Just so the history of diacetyl claims/testing for the major manufacturers is all in one place. These are the ones I know about:
Flavor West lied. They said they had no diacetyl in any flavors, but a juice vendor in Canada doubted that and sent a sample of FA Butterscotch to a lab. Analysis revealed massive quantities of diacetyl in there. FW has been removing the 'no diacetyl' claim from some flavorings. Meanwhile FW produced a short list of flavors they said had some diacetyl, but the list didn't include butterscotch.
Inawera lied. They told numerous people in personal emails that their products contained no diacetyl, no acetyl propionyl... They very recently produced a short list of flavors they say have some diacetyl: ""coffe, tiramisu, cappuccino, cocopilada knows tabacco, TOBACCO "APPLE", SHISHA FLAVOUR VANILLA TYPE, TABACCO "CAPPUCCINO" It was discussed
here, partially in Dutch.
LorAnn lied. They used to claim that none of their products contained any diacetyl. That claim was on their website FAQ for a while. By late 2013 that claim disappeared. Next they said they could provide a list of flavors that contain it, but to my knowledge, they never did. Now if you contact them regarding ingredients they will tell you their products are not intended for inhalation.
Hangsen gives conflicting information to people who ask about their ingredients, depending on whom you ask. I don't what their current claim is.
Capella never denied the diacetyl in their flavorings, so far as I know. Given that they're now making 'version 2' of some flavors popular with vapers, and they claim v2 has no diacetyl, then anything not v2 may well contain it.
Nature's Flavors, when asked about diacetyl in the product line used for vaping, refuse to say; they do tell you their flavorings aren't designed for inhalation. They do have a product line specifically advertised as diacetyl-free, but those are gum arabic flavors that can't be used for vaping. The ones used by vapers do not claim to be diacetyl-free.
The Flavor Apprentice tests their flavorings and
publishes results here. (click 'list' on the right of any flavor name) That info is incomplete and subject to frequent changes, but they are the only flavoring manufacturer who does this. The minimum amount they report is "<0.5%."
FlavourArt has a Kitchen Magic and an E-Cig line. Kitchen Magic flavors with diacetyl show an inhalation warning. Their E-Cig flavors (listed on their
main website) have no diacetyl or related diketones. They used to list percents of diacetyl for Kitchen flavors that contained it, but now they just have the warning (and are not listed as an E-Cig flavor). Even flavors with less than 0.0012% diacetyl were removed from their E-Cig line. No other manufacturer tests with near this much precision.
One thing to be aware of: In the U.S., any product with <1% diacetyl can legally be labeled "diacetyl-free." I believe the cutoff for acetyl propionyl is also 1%. So a product can legally claim it has none while containing nearly 2%. To put that number in perspective, FlavourArt's Butter flavoring (has inhalation warning, not in their e-cig line) contains 1.5% diacetyl.