Here are my thoughts based on my own experience, and they are worth what was paid for them. LOL
I didn't try to stop smoking. Instead, I continued smoking 5 analogs a day and used vaping for the rest. The reason is because I don't have the luxury of downtime. I'm a mommy, an author, a scientist, a computer consultant, etc. and people are always demanding that I give my best. I simply do not have the luxury of being able to perform suboptimally even for a day.
I had been smoking 25-30 cigarettes a day. I replaced all but five of them with vaping.
Now, I realize the surgeon general and all those guys draw absolutely no distinction between the amount of harm that will come from second-hand smoke smelled from 30 yards away and smoking 3 packs a day. But I consider myself to be smarter than them, and it makes sense to me that LESS tobacco is better than MORE tobacco. You do not have to completely replace tobacco to realize a health benefit -- the health benefit is going to be proportional to the amount of tobacco you replace.
I mean, seriously, would any rational person (in which group I do not include the surgeon general et al) believe that the potential for harm from 35 cigs a week is identical to the potential for harm from 200 cigs a week? Granted, you COULD get cancer in either case; but I am quite certain there is a difference in ODDS.
I'm not someone who smoked heavily for 40 years, but even so in switching from 200 cigs a week to 35 I experienced immediate tangible positive benefits and WITHOUT the dramatic negative impact on my cognitive capacities that I have experienced when trying to quit smoking altogether.
So I don't see this as an all or nothing thing -- every analog avoided is a positive. You don't have to avoid all analogs to experience benefits. You could even use vaping to replace just every other analog. It would still be worth doing.
That's my first thought.
My second is an observation. When I first started vaping, I needed 18mg-24mg juice. Initially, that's what was really needed to get rid of cravings for analogs. But then, as time went on, whether it was due to more efficient uptake of the nicotine, better vaping gear or whatnot, I started to experience OD symptoms from that much nic. Tiredness, sleepiness, headache, tachycardia, etc.
I make my own juice, so I started knocking it down. Now, even though I am still using 5 analogs a day, I am using 10mg juice instead of 24 where I started.
At first, I thought that maybe the place where I bought my nic had mislabeled 100mg nic as 60mg; but I tested it and it was 60; so I had simply become more sensitive to the nicotine and required less.
I have not explored this phenomenon in great depth so I do not know its cause, but I suspect this is fairly common.
So my second thought is: if you are having symptoms of too much nicotine, decrease your nicotine intake.