This is my current stance on vaping eliquids with trace amounts of diketones. I haven't completely culled my suppliers that still have a couple with trace amounts. Using The Plume Room as an example. Like Charlie Noble a few of those have trace amounts of AP or Acetoin ..not more than 300ppm per ml and that's a very small amount:
According to NIOSH the safe limit is 3-7ppb so I feel safe in my decision to vape those in moderation.
Eventually I will go 100% diketones free, but as long as TPR and Charlie Noble are giving me the heads up on just how much is in the flavors that do have trace amounts, I feel comfortable vaping that suppliers eliquids.
http://www.cdc.gov/niosh/topics/flavorings/exposure.html
remind me again how ppb translates to ug/ml?