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. It takes a ton of breath, breath support and a bit of micromanaging.
In this variation on the familiar exercise, the singer moves between two closed vowels. The way we did it was to move from either extreme ([i] or [u]) and then move to the next closed vowel ([e] or [o], respectively). We would also move in an “up a fourth” “down a third” movement so that the voice never sat too long in one range.
What you are aiming for is the sensation that your voice “locks” into some place that feels better than the others. In that place you can easily crescendo and decrescendo.