You can try to educate your girlfriend. My husband did not want to vape (not for safety issues, he thought it was dumb.) Well, I was enthusiastic after I had quit, and did a bit of nagging.
A few months later (no nagging, just me vaping. Continually, Doing great at it) he stated he wanted to start vaping. IDK if he'll ever get past dual use. What you can do is vape (I might do some initial educating, but if she's too scared, she may not be ready to hear it). So, if she becomes a smoker again, keep vaping, don't nag, and show her it is safe by vaping around her as much as possible. Really, sometimes demonstrating things can be the best way to go about it.
Good luck,
Anna