Are water vapors without nicotine dangerous ?

Depends: on if any other chemicals are mixed in the water vapor.

Not in that way: The only danger that I can think of would be bacterial or viral infection. Moist, warm conditions are fantastic breeding ground for them. If you clean - very hot or boiling water and/or alcohol - the device before use and don't share w/ others who might carry disease it sounds safe. Even this may be a bit over-cautious for a strong, healthy young adult, but safe is better than sorry for fun things