kevintech, I don't really feel that anyone on this forum can give you a straight-up and honest answer on what to do here. We're not involved in the situation, we don't understand when you two are honestly coming from. Sounds like some of us have had some bad turnouts coming from your situation and some of us (well, a lot less of us though) have had some good ones. Doesn't really mean any of us know where you're coming from though, 8 years of experiences with another person are hard to sum up in an internet post, right?
I do know, if it was me, I'd let her know I care about her. I'd bring her flowers, still keep trying to be with her, but I also wouldn't quit vaping. I wouldn't lie about it, but if she wants to make the decision to end your relationship, that is her deciding it. I would say 8 years is an extremely long commitment to give up on; and thats aimed at both ends. All you can do is be who you are, keep caring for her, and not let go until she makes you. This isn't a case of marijuana vs girl; its a case of the girl trying to change your habits. Just let her know you don't see it as an ultimatum and maybe she'll stop seeing it that way too.
And maybe take a note out of momofthegoon's book. Traits like "thin", "good-looking", "all the other good ones are taken" and "never cheat on me" aren't the things to tote you got out of a long-term relationship (not to say a little mutual play with such traits is wrong, its just obviously past that point). Look for "loves", "commits", "works with me", and "makes me want to be a better person" to really choose a girl to keep around. If its 8 years you should know if you love each other or not. If you really do, then no one would be talking about leaving over such a small thing. If you don't, then it shouldn't be too much of an issue.
Sidenote: There might be more personal reasons she wants you to quit. The only time I've felt such an ultimatum was necessary would be situations similar to one of my older best friends. His fiance had been abused as a child by her scum family. They'd get high, and oftentimes force her to get high around the age of 8 too. Something like that happened to me I'd hate weed too.