Analogs contain approximately 9 mg of nicotine , although we only absorb about 1-2 mg of nic per cig: http://www.ehow.com/about_5370963_much-nicotine-one-cigarette.html
So if you smoke a pack a day you are getting 20-40mg of nic a day. If you are vaping 2 ml of 18mg/ml of e-juice a day, you are getting about the same amount of nic.