[This is an updated post that was first posted several months ago.]
This is a vocalise that my earlier teacher Dr. Day taught me. He never presented it as being hard or anything. It was just the relaxing vocalise we did at the end of every warm-up. It also probably helped that he played pretty plagal cadences underneath me (as I have notated in the piano part).
Of course, the messa di voce is hard. By increasing air pressure to crescendo (get louder) you may accidentally cause a rise in pitch. The opposite is also true. Therefore, several adjustments must happen all at once while you crescendo and decrescendo (get softer).