For a user: piezo buzzer receives electric power at its resonant frequency and emits a sound at this frequency. It can be used as a microphone, where it produces an electric signal at the sound frequency, which must be at resonance.
The buzzer and sensor must hence have resonant frequencies near to an other, which suplpiers take care of. Or you use the same part to transmit first, receive later.
Water in the vapour, you mean a little bit of fog where you want dry vapour?
This should increase the acoustic losses of the vapour. Reduce sound speed slightly as well. Both are measureable, I ignore which one is used. With a single buzzer transmitting permanently, the electric impedance will vary as its cavity is damped and de-tuned.
Not obvious: most buzzers resonate a ceramic disk AND an acoustic cavity, and are tuned for air. Vapour will de-tune the acoustic cavity a lot. Modify it for vapour? It will depend on temperature. Use a buzzer without an acoustic cavity?