In moderation, vapes are not bad for you. In excess, they can pose a problem. To put it into perspective, too much water can kill someone, though too much vaping will most likely only produce temporary side effects that may be unpleasant.
Vape Pens are actually intended to replace harsh smoking methods, which are known to be much more harmful to people. Vapes are only as bad as the people who use them, as developing a habit from someone with an addictive personality will abuse such devices.