Vaping is completely different to smoking, we can try and compare it all we want but the truth is its completely subjective, to the quality of the ejuice and equipment your using, the way you inhale, and differences in tolerance form person to person.
for a start, the absorption of the nicotine when vaping, is much slower, 30 secs to get to the brain, as compared to 8 seconds from smoke. vapour droplets are significantly larger in size than smoke, so they are not able to penetrate the lungs as deeply, and lung absorptions is probably equal or less to that in the mouth.
to absorb the most out of puff, its best to slowly drag into the mouth, hold it there a second or 2, inhale the vapur into the lungs, then breathe it out through the nose. a combination of the mouth, lung and nasal absortion areas will likely get you the most nic absorption
most experienced vapers say around 3-4 drops of eliquid is equal to a cigarette. so 4ml's worth of juice is around 15-20 cigarettes in terms of number of puffs. the 18mg nicotine is suppose to be a medium strength 0.8-1mg cigarette.
though newcomers to vaping will have a tendency to over compensate with a higher nicotine strength and intake, to compensate for the lack of MAOI's they use to get from cigarettes.