The flavor is the biggest unknown. Especially if the particular flavor used is unknown.
I personally mixed my own juices, and all I can say with regard to flavors is that the amounts are very small for most flavors, and vaping it is not much different from smelling it if added to food or even tea as flavoring.
So if these flavorings are a significant health hazard, I think it would have been detected in the industrial processes where they are used, and in professional kitchens.
There was the infamous popcorn lung situations, where workers on a factory making artificial butter flavor were inhaling crazy high amounts of it. Like millions of times what you would inhale when vaping. It was called popcorn lungs, because the butter flavoring was used for microwave popcorn.
It caused enormous damage to the reputation of this type of flavoring, but in reality the flavor wasn’t really the issue, it was the insanely bad and irresponsible conditions under which it was produced, that offered absolutely zero protection for the workers, while there also was near zero ventilation, causing the flavor to permeate the production facility in extreme concentrations.