I don't think any of the reputable e-liquid manufacturers still use Diacetyl do they? At least the brand I commonly use (Dekang) explicitly claims they don't.
FAQ
I like how they list the ingredients they use (including flavoring) on their juice bottles along with the percentages contained. Whether it's accurate or not I don't know for sure. I don't know what these flavor components are anyway.
Well... as far as I know, there's no safer substitute for it?? It IS the butter flavor... horse of a different color.
Lets look at the only substitute to it, Pentanedione.
So here's what the CDC has to say about both:
CDC - Flavorings-Related Lung Disease: Exposures to Flavoring Chemicals - NIOSH Workplace Safety and Health Topic
Effects on rats for both:
diacetyl
Subsequent studies have helped to clarify the role of diacetyl. Toxicology studies have shown that vapors from heated butter flavorings can cause damage to airways in animals (Hubbs et al, 2002). Studies in both rats and mice demonstrate that the cells lining airways can be damaged by inhaling diacetyl vapors as a single agent exposure in both acute and subchronic studies (Hubbs et al, 2008; Morgan et al, 2008). In mice, aspiration of diacetyl alone caused a pattern of injury that replicates some of the features of human obliterative bronchiolitis (Morgan et al, 2008).
Pentanedione
The alpha-diketone, 2,3-pentanedione, has received attention as a flavoring substitute for diacetyl. It is also known as acetyl propionyl or by CAS number 600-14-6. It is structurally very similar to diacetyl since 2,3-pentanedione is a 5-carbon alpha-diketone and diacetyl is a 4-carbon alpha-diketone. Published reports on the toxicity of 2,3-pentanedione were first published in abstract form in 2010 (Hubbset al. 2010b; Morgan et al. 2010). A recent NIOSH peer-reviewed publication documents that acute inhalation exposures to 2,3-pentanedione cause airway epithelial damage that is similar to diacetyl in laboratory studies (Hubbset al. 2012). In 2-week inhalation studies in rats, NIEHS researchers found that 2,3-pentanedione caused proliferation of fibrous connective tissue in the walls of airways and projections of fibrous connective tissue sometimes extended into the air passageways (Morganet al. 2012b). Preliminary data suggest that repeated exposures to either 2,3-pentanedione or diacetyl can cause airway fibrosis in rats (Morganet al. 2012a). In the acute inhalation study of 2,3-pentanedione, changes in gene expression were noted in the brain (Hubbset al. 2012). Preliminary data suggests that diacetyl can cause changes in the central nervous system that are similar to those caused by 2,3-pentanedione (Hubbset al. 2010a). As a group, these publications raise concerns that the toxicologic effects of diacetyl may be shared with alpha-diketones which are close structural analogs.
Sooo... they have the same effect. So why use Pentanedione as a substitute for diacetyl? Because... THEY HAVEN"T BEEN SUED FOR IT YET. It's a new substitute... that does exactly the same thing to the human body.
Note that Osha doesn't care what the substitute is... it's warning applies to ANY butter flavor:
https://www.osha.gov/SLTC/flavoringlung/diacetyl_worker_alert.html
How about not vaping butter

?