Vaping is - according to Public Health England - 95% safer than smoking, and popcorn lung is linked to diacetyl, a flavouring ingredient which is not now used in vaping liquids. Nicotine (although addictive) is no more dangerous than caffeine - it's the other ingredients in tobacco smoke and cigarettes that will harm you. Please ignore the ill-informed post above - I wish people would research a little before making incorrect statements.
Yes, you can go from smoking 20 a day to vaping - I've done it. There are vaping topics over on the Stop Smoking board where you could ask questions, and get answers from people who actually know what they're talking about :)