The same study also looked at non-diacetyl and non-acetyl propionyl containing e-liquids being spiked with diacetyl and acetyl propionyl and the concentrations of these chemicals in the e-liquid after 18 days. The results showed that the levels of each chemical dropped to well under half of the starting level after this time period, indicating that both are unstable in e-liquids, perhaps reacting with the VG and PG to form chemicals known as ketals and hemiketals until an equilibrium is reached.
Together, these two experiments indicate that any e-liquid containing acetoin, especially those with nicotine, will form a small amount diacetyl and that some of this diacetly will further breakdown into different chemicals until an equilibrium between the three groups is reached. Eliminating diacetyl entirely is thus super difficult (it's possible other chemicals present in flavours could do similar things), however by using ingredients with no added diacetly the levels of diacetyl in the finished juice will be very very small traces, perhaps even below definitively measurable levels.
1. 'The Stability of Diacetyl and Acetyl Propionyl and the Conversion of Acetoin to Diacetyl in e-liquids' Vas, C., et al., http://www.bat-science.com/