Up to date, the only way to create "clouds of vapor" is by using a wick and coil.
The ultrasonic transducers that are commercially available right now are the weak ones that you commonly see in household nebulizers. Those wispy vapor machines used for aroma therapy.
But an ultra-powerful one will revolutionize vaping and will eliminate the need for a wick and coil. Effectively making everything that we use now ...obsolete.
I foresee potential problems with extremely powerful ultrasonic anything; it's known that subsonics have an impact on the brain, inducing fear; if subsonics can do that, what can ultrasonics do? I agree, the ones commercially available right now are very weak, but we really have no idea what much stronger ultrasonics might do to us, physiologically speaking, and it bears looking into before being introduced widely. Sounds do have an effect on humans, and clearly, even sounds that we don't actually hear -- subsonics can be felt, sometimes, but not strictly "heard".
Andria