If a vacuum is being created by cavitation then in essence the water is being vacuum boiled. Some dissolved solids will come out of solution. A NaCl molecule has an atomic weight of 28 vs the 10 of an H20 molecule. Sodium alone weighs in at 11. Then there are other common dissolved solids such as calcium (atomic weight 20).
A major health problem with ultrasonic humidifiers is that some bacteria in the water also gets dispersed along with the water vapor. So some undissolved particles would also get dispersed.