From the other end of the spectrum...
30 year 2 PAD ex-smoker. I continued to smoke during my first 90 days of vaping. I had the cigarettes on hand since I had just bought my usual 1 month supply. And I couldn't in good conscience give them away. ("Here, have a box of carcinogen.") Anyway, the transition wasn't as easy for me as some people. Those first two weeks were kinda rough, brain fog, quit smoking withdrawal. Still, it was a lot easier than with all the pharma approved attempts in the past.
Right away I dropped from my 2 PAD habit to 10 butts, to 6, to 6, to 4... The rest of those first three months were like that. Some days I didn't have any at all. A bad day was 6 or 7. As things moved along I disliked smoking more and more. A lot of us go through that. In it's own weird way, smoking a cigarette turned into it's own worst negative reinforcement.
Ultimately it came down to: 30 year habit, 3 months to completely quit. I didn't see a problem there. That flies in the face of everything you have been told to do by the ACS to quit. And we all know how well trying to do it their way works out... I was down to my last .... in my last pack, smoked it, didn't enjoy it at all - and it was over.
My point is there are no rules to quitting with e-cigs. You get to figure out your own end game. Smoking while transitioning is not an automatic failure. As long as you have the desire and motivation to quit you can do it the way that works for you.