They claim it doesn't but tests have
shown otherwise.
I did a little digging to see what I could find on Capella's Vanilla Custard and managed to pull this from another forum. Apparently Tom, from Capella's, sent a sample of their Vanilla Custard to a lab to have it tested and analyzed due to the questions from the vaping community regarding Diacetyl's presence (the company is ABC - Advanced Botanical Consulting).
The guy that posted it mentioned speaking to a chemist that worked on energy drinks to see if the report was anything to be concerned about, given the result was < .4% and he said that from speaking to him, those are the tolerances that chemists work in and that result indicates that Diacetyl is
not present, otherwise it'd show > .4% with the < or > being the key indicator.
I've attached a screenshot of the report below to confirm this. That said, I'm not a chemist, nor do I claim to be one. Just as the above is second-hand info, so is this and the information above. Take it for what it's worth and make your own decisions as to whether you want to use it or not. Personally, I've felt no ill effects, but that doesn't mean there's none. I do, however, feel that I'm exposed to far worse walking outdoors than I do adding a % to my e-liquid.