Honestly these things depend on the individual. For instance my brother and I both smoked for probably 15 years and we both got started on vaping at the same time, even went to the shop and bought our starter kits at the same time. It's been months and I'm almost exclusively vaping and he's still smoking. I've found that the flavors drew me in because it tastes better than a cigarette. My brother disagrees and is the opposite. In his defense though, he does use his stuff sometimes though, I'd estimate a 10-20% drop in cigarettes smoked depending on the day. Sometimes he vapes all day and sometimes he doesn't touch it. Like yesterday when we we're hanging out he was puffing on this root beer flavor all night and didn't go out for a smoke at all. Today I filled his EMOW with some new MBV juice i just got today and he didn't touch it once. But as long as he's replacing at least a few smokes with vaping i consider it a victory.
I do worry about his health, he had a mass removed from his thyroid last year, everything went okay and he's fine, but that would have scared the .... out of me if it was me. Hell it did and it wasn't me. I can only provide the tastiest possible juices and provide an example, but I can't make him quit and that's fine with me.
I occasionally sneak in a smoke when he goes out for one though. But in my defense i rarely smoke more than one cigarette a day anymore. And even then I get halfway through and just want my vaporizer lol.