diacetyl is present in cigarettes at 700x the level of any vaping liquid containing it[1]; besides that there is no actual documented case of a vaper (or smoker) developing popcorn lung; afaik this has only happened to some employees at a popcorn factory. as others in the thread have mentioned, almost all juice manufacturers moved away from using diacetyl after this scare arose a few years ago.

[1] https://tobaccoanalysis.blogspot.com/2015/12/new-study-finds...