I think the whole discussion is moot, because when you heat VG it produces acetoin, which reacts with other chemicals to produce diacetyl. So, even if you test the liquid and it contains under 1pbb of diacetyl, it still produces diacetyl when it's heated. Now, the actual temperature at which the reaction from VG to acetoin occurs is above where most of us consider a good vape, so as a practical consideration, this is not a major health consideration. However, the early studies that FDA is working from heated the VG to over 400C, and found significant quantities of acetoin, aldehydes, and lots of other nasty stuff. Based on that data, it is flat-out unsafe to vape pure VG, let alone VG plus the other stuff we mix with it. So, flat out, the FDA believes that all vaping is a health hazard.
Now some of you seem to be operating under the mistaken belief that the FDA's job is to only let through chemicals that are 'mostly safe.' That hasn't been true since at least the 60's, when large-scale testing of thalidomide led to over a dozen infants being born with birth defects. Since at least that time, the FDA's charter has been to only let chemicals through that are completely safe. And, vaping VG is not completely safe.