I was a smoker for 19 years, started at 1 pack (20 cigs) a day and when i quit i was at around 2 packs+ (40 cigs), i quit using vapes for 3 or 4 years and i havent touched a cigarrette, however i learned a lot about what happens when you switch, ill share a few pointers.
1) Some people can stop smoking easily, some like me have a helll of a time, seems there is an enzime in the liver and everyone processes how much nicotine actually ends up in the brain differently, a lack of the enzime will me you an ultra addict to it, high ammounts of the enzime will make it so less nicotine is actually reaching your brain and maybe you smoke 20 cigs but only 5 will reach the brain in ammount of nic, thats why for some people is easier and some suffer.
2) Use common sense, i seen a lot of people (friends and family) start and order... 0 mg (no nic) liquid... how do you expect to jump from 20 - 40 cigs a day to 0 nic is beyond me but when asked they would say "i dont want nicotine anymore", yes i know but you brain is ADDICTED to it, your brain will demand it, you need to pick a nic strenght that matches your current nictoine consumption and lower it slowly with vaping over xx ammount of time (different people different ammounts), for example if you smoke 1 pack (20) of high nic cigarretes (like marlboros, luckies, etc) you need to at least start at 18 mg, if you start at 0 you will just go back to cigs.
3) Do not, i repeat, do not mix the 2 things, you either smoke or vape, when you smoke the defenses of your body are put on overload all the time specially on the lungs, regular cigarrettes shut down a lot of systems on the body and that is why tar accumulates along with other stuff, the body cant keep up, plus you run the risk of actually increasing the ammount of nicotine on the body since vaping is usually high on nic to compensate the delivery method (it takes longer for the nic to reach the brain when vaping), do a full switch to vaping to let the body recover, depends on the time you have been a smoker it can take 2 or 3 months, you will still be coughing out gunk, specially once the body starts to be smoke free again, you will recover taste, breathing capacity, etc slowly and will cough out gray/brown gunk that was on your lungs for years, this is normal.
4) Do not go cheap on the vaping gear, aim for good quality mods (smok alien, joyetech espion, etc), yes yes i know these can be sometimes bulky but the experience is completely fullfilling, be thankfull you didnt have to quit using egos, they where usefull but man you try these new mods and is a whole different world, a typical good quality mod should go for $60 us.
5) I been smoke free for 7 years, was declared non smoker by my ensurance only like 3 years ago (much to my old doc surprise lol), it takes a looong time for the body to remove the damage of smoking, but is amazing, be strong!!, im to slowly lower the nic ammount in your vape liquids as you feel fit dont cut too much to fast, dont stay years on same nic, avoid nic salts (extremly high nic for a marginal "better" cig emulation).