I wish I could find you one of the many threads that discuss how much nicotine you are getting from vaping versus from a real cigarette, but those threads are very hard to search for.

Suffice to say it is a complicated.
One thing to know, for starters, is that there are studies that show we only absorb about 10% of the nicotine from vapor as compared to cigarette smoke. And another thing to know is that 18mg means 18mg/ml so that means you would be using 18mg of nicotine in one milliliter of juice. Most people use around 2-3ml per day of juice, so keep that in mind too.
My gut feel is that you might want to try 11-12mg to start with.
But you never said how MANY of those things you smoked per day, which matters quite a bit.
Suffice to say it is a complicated.
One thing to know, for starters, is that there are studies that show we only absorb about 10% of the nicotine from vapor as compared to cigarette smoke. And another thing to know is that 18mg means 18mg/ml so that means you would be using 18mg of nicotine in one milliliter of juice. Most people use around 2-3ml per day of juice, so keep that in mind too.
My gut feel is that you might want to try 11-12mg to start with.
But you never said how MANY of those things you smoked per day, which matters quite a bit.