Nictonine is not harmless, but e-cigarettes are, and indeed, MUST, be safer than ordinary cigarettes. It just stands to reason - the most harmful substances in cigarettes are NOT the nicotine, but the 300+ carcinogens that are created by burning the tobacco, tar and paper.
E-cigarettes would have, logically, fewer toxic substances - in fact, the ONLY substance they give to the user, except for water vapour, is nicotine! That's literally all it's giving to the user - Nicotine.
Now, nicotine is extremely addictive and far from harmless - it has nasty impacts on blood pressure, for instance. But there's no way in hell it could be anywhere near as harmful as the ordinary, tobacco based cigarettes. Even if we don't know the long-term health effects in detail, we already know enough to know that they're far less harmful than the 'real' things.
I would encourage smokers to quit entirely - nicotine is not harmless and it is EXTREMELY addictive. As a general rule, it's a bad idea to be addicted to anything. Never let a substance run your life (and no, we're not 'addicted' to water - water is a necessity, nicotine is not).
But if you're not going to quit - vape. Seriously. Do it - you'll be doing far less damage to your lungs. It still wouldn't be good for you, but it's a hell of a lot better than using cigarettes filled with tobacco and paper and tar.