I'd second the advice about maybe trying 9mg.
The advice that "you'll know when you've had enough" is not necessarily true, because vaporized nicotine takes a great deal longer to reach the brain than nicotine in smoke; by the time you feel sick, you could have vaped for 15 mins past where you'd actually had enough -- I had that problem with 18mg and 12mg.
First sign of too much is usually, feeling kinda nervous/hyper; then little sharp pins and needles pain all over your abdomen; maybe a bit of light-headedness, head spins, dizziness; maybe headache; then nausea, which could progress to vomiting.
The method of inhalation is very different from smoking, to get max nicotine ASAP. Smoking, you drew it deep into your lungs, and that is not very effective with vapor; hold it in your mouth and throat for a bit before inhaling, then exhale via your nose as much as your mouth -- that's how you get max nicotine from vapor, but it will still take longer than smoke.
Andria