Obviously this is a somewhat controversial topic, so OP was never going to get a good, clear answer from a public forum.
I don’t don’t vape, and haven’t smoked cigarettes since I was like 15. Cigarettes, like alcohol, are one of those drugs where their lasting effects and risks heighten with consecutive and frequent use, hence “overdosing”. Multiple studies have found vaping to be harmless in people, you can literally make your own juice at home if you were ever concerned about what’s in it. Regardless of the studies, it’s unarguable that vaping is Immeasurably better for you (the smoker, their environment, their kids, their pets, etc) than smoking, so handing people articles saying ‘oh, read this, it’ll tell you why to stop’ isn’t helpful when they’ve adopted the least harmful possible smoking practice available today.
As for the rat side of things, the biggest concern is likely going to be oxygen availability and scent, though perhaps a vet will have better info and I’d approach them before a forum, personally. If I expect any heavy scent in my rat’s space, then I open the window open that’s behind them so help waft the air around the so nothing ‘hangs’. I imagine that flavourless juice will be less irritating to rats than flavoured as some juices leave a lot of scent in the cloud, but maybe certain flavoured juices won’t do that (I haven’t seen or looked for any studies about rate of vapour dispersal), or you can find a vape that lets you control the cloud size. If you use flavoured juice but vape in other rooms, maybe don’t handle your rats right after vaping to allow the smell of your hands and clothes to neutralise a bit?
obviously it’s all down to common sense; if you started vaping as an alternative to smoking, then you’re benefiting your rats. If you take measures to control what potential dangers enter their space (toxic foods, strong smell, beady-eyed cats, etc etc etc) then you’re benefitting your rats.
