I'd agree that contacting the manufacturer is your best bet.
The problem with that approach is that some manufacturers were caught lying when asked questions like that in years gone by.
Also, asking about diacetyl is the wrong question. When the community became aware of the potential dangers associated with this flavoring component some years ago, lots of manufacturers did pull it, and replaced it with acetyl propionyl, which has a very similar flavor profile. Unfortunately, acetyl propionyl also has very similar effects on lung tissue in animal studies. This means a vendor could truthfully answer, "Nope, no diacetyl in our liquids" while selling you something that has essentially the same risks.
If someone is worried about this -- and I'm not saying anyone should or should not be -- IMO, the right question to ask is whether the vendor has had their liquids tested for diketones, which covers both. If they claim they have had them tested, then ask 'em to see the results.
Personally, I no longer worry about this stuff because I use almost negligible amounts of flavorings, and I was never a fan of the flavor profiles that are likely to have high levels of the bad stuff to begin with.