Vaping is not smoking. Because the nicotine is not floating on miniscule carbon molecules (Smoke) it is actually entering your system prior to it hitting your lungs. With vaping, the nicotine is absorbed quickly through the flesh of your mouth, your throat, your esophagus. So unlike smoking, the nicotine is being absorbed orally, & entering your bloodstream through what psychoactive substance researchers like to call the ORA, or the "Oral Rout of Administration"
While it does get absorbed faster orally, it does not make a complete circulatory journey as quickly as it would had it been absorbed through the lungs. This simply means that with vaping, it takes our bodies longer to feel the effects of the nicotine drug on our central nervous systems. So, we don't get the same "rush" that we would have had we been smoking. The nicotine in our vapor has more of a trickle effect, slowly being absorbed, slowly hitting our blood stream, slowly passing the blood brain barrier, and slowly effecting our CNS.
So, the average cigarette might have less nicotine by volume, but it is absorbed and utilized more efficiently than with vapor. 18mg juice is the closest mg range to best replicate the experience of a smoked cigarette. 18 to 24mg nicotine is much higher than the amount in a cigarette, but with that large amount of nicotine in the vapor, it will make up for the slower and less efficient oral absorption.
While this is true, taking a puff from an e-cig will still not effect your body as immediately as a cigarette would... And at the same time, the nicotine will be utilized more slowly. It will have a longer half-life once it has entered our bodies. This means that while we might feel like we are vaping much more than we were smoking, we are (depending on the juice and your pace) for the most part, never reaching the highest peak nicotine dose to brain peaks that would be experienced by smoking. However, the baseline lasts longer. So a 3, 4, 5, or 10 minute vape session can keep your body from physically withdrawing from nicotine much longer than having 1 cigarette... BUT, you will still crave those peak levels much more after a vape session, than you would had you just smoked a cigarette.
So vaping is kind-of a whole new monster. Its not as easy to reach those peaks and get that light-headed feeling as it was with cigarettes because rather than hitting us all at once with cigs, with vaping we are slowwwwly absorbing our nicotine. If you want to best replicate smoking, I suggest a mixture of two different juice levels. For every day, all day type vaping, I recommend 6, or 11-12mg juice. But when you are truly craving that buzz, (technically, that "peak" level of nicotine that smoke allows you to reach) you should have a cartomizer or atomizer that you can use with 18 to 24mg juice.
I personally use 12mg juice regularly and I will add 18mg flavorless juice to it in an atomizer when I am truly craving a nicotine buzz. Naturally, the further your two mg amounts are... (all day VS. buzz giving) the better the buzz of the buzz giving juice will supply you. Therefore, if you were to use 6mg juice on a regular basis and use 18mg juice to catch that peak, the peak would be more easily obtained than had you been using 12mg juice all day. This is how the body, and the cycle of "withdrawal & high" of an addict works.