With regards to the flavourings, if you are using a vaping supplier you should be avoiding the most dangerous one OIL.
After that the known risky ones are Diacetyl and it's little friends. Diacetyl has been known about for a while and you will come across flavours certified as diacetyl free. The problem is that sometimes the diacetyl has been replaced with one of its little companions which whilst not diacetyl itself can break down into diacetyl when exposed to HEAT. The dangers of the companion chemicals like acetoin and acetyl propionyl haven't been known about for as long as diacetyl, so the industry is still playing catch up. The end result of this is at the moment it is pretty difficult to avoid vaping the stuff. It tends to be used in creamy flavours and buttery ones so you might reduce your chances of coming across it by avoiding these.
However as you rightly say it is in cigarette smoke and generally in far larger quantities than it is in eliquid so it is not exactly a new chemical you would be introducing to your system, just one that with care and attention by the suppliers we should be able to avoid in future. Problem is that it is going to take time to get there.
