1) They are loaded with chemicals known to cause harmful effects
2) It's not "just water vapor" that is exhaled by users
3) Users insist on using them indoors and in close proximity to others (see #2)
4) The large lobbying effort to make them socially acceptable often straight-up lies about their effects (again, see #2)
Just about the only "Good" thing about them is the lack of carbon being inhaled and released into the air (smoke)... otherwise they are just as bad as plain old cigarettes. What we have done is taken the Tobacco leaf + additives and removed the Tobacco leaf, leaving only the additives.
The lack of scent in the air doesn't mean the harmful "second-hand" chemicals are not present... it just means the tell-tale smell of burning paper and tobacco isn't present, making E-Cigarettes more dangerous to bystanders (no sensory warning signs to detect).
Regarding the lie that is #2, it if really was "just water vapor" being exhaled, you would not have inhaled it in the first place (there would be no effect). The lungs do not 100% process all of the chemicals being inhaled, which means you are exhaling them into the air just the same.