When would you use a vaporizer vs using a humidifier? which would you use to constantly maintain a clean aired baby room ?

Humidity: Both are effective for increasing humidity in a room. Vaporizers tend to create environment that can promote growth of bacteria, so you have to make sure you clean it daily and change water. Vaporizers also can be dangerous to toddlers and small children as the steam can cause burns if touched by little hands. Neither is going to clean the air. You need an air purifier for this.