To the best of my knowledge there’s a lot of information in the extremes- that it’s either harmless or right up there with chugging cigarettes, and not a whole lot in the middle nor consideration for variables like quality between manufacturers, ingredients, etc. At the end of the day, if people were meant to inhale vaporized propylene glycol and nicotine, they wouldn’t catch a buzz from it and something like it would exist in nature, so just assume it’s at least not great and accept the risk of harm if you choose to do it. At least until more concrete evidence presents itself.