Regarding flavor safety:
Any sweetners, artificial flavors, or e juices with no ingedient listings are probably bad news or no news which is just as bad, IMO.
However, some are perfectly safe.
Vanilin, for example, is very unlikely to cause any damage. It's a very natural compound, the kind people inhale anytime they use essential oils, because it is an essential oil. I dunno if I would put ethyl maltol in the same boat because I am unfamiliar with it. Though, I can't say I enjoy it overly much.