Health is a component of Veganism, but technically if someone avoids animal products purely for health they are not really Vegans. The term Vegan was first coined in 1944 by an Englishman named Donald Watson, who defined it as such:
"Veganism denotes a philosophy and way of living which seeks to exclude - as far as possible and practical - all forms of exploitation of and cruelty to, animals for food, clothing; or any other purpose; and by extension promotes the development and use of alternatives for the benefit of humans, animals, and the environment."
The underlying motivation for Veganism is ethical, not health. Therefore a person can smoke tobacco, provided it is pure (preferably organic) and not laced with animals products (which many cigarettes are). Personally as a Vegan, I don't smoke, but technically I could and still be a Vegan.