I agree with the above comments that the major health risks with cigarettes is the smoke created from the combustion of tobacco, and the hundreds of other chemicals created from that combustion (cyanide, arsenic, carbon monoxide, etc). It is this scenario which causes lung and oral cancer, chronic obstructive pulmonary disease, and chronic bronchitis.
The health risks of inhaling vaporized vegetable glycerin and propylene glycol have yet to be determined by long term studies, but it is widely believed to be far less harmful than cigarette smoke.
Nicotine is both a stimulant and depressant. There are some pro's and con's to using nicotine. Much depends upon the dosage or quantity consumed. Nicotine is a stimulant much like caffeine, and their effects are comparative. That's why many users search for that "buzz" they get from both a cigarette and an e-cigarette, and caffeine products.
Nicotine does have medical side effects. It is a vasoconstrictor of the smaller blood vessels (capillaries), so if you have heart disease, coronary heart disease, hypertension, arterial vascular disease, peripheral vascular disease, or diabetes, it could cause these conditions to worsen. It can also negatively effect the outcome of certain surgical procedures. Plastic/reconstructive surgeons will not perform elective procedures on someone who smokes or uses nicotine, as this would negatively effect the healing process of fragile tissues. Nicotine also prohibits new bone growth, so orthopaedic/spinal surgeons will no do elective bone and joint surgeries on patients who smoke or use nicotine products.
I'm a retired RN and physician's assistant, who worked over 35 years in primary care and in the operating room.