Here’s a different way to look at “vowel modification” – the goal is not to modify the vowel, but to modify the way we make each vowel so it sounds the same throughout the range.
Singing is communication, and the vowels carry the meaning and beauty of any language. If the vowels are generalized or incorrect, then communication, meaning, and beauty are all lost.
But, you say, what about vowel modification? All those charts? Well, the voice does change throughout the range. If you kept the same shape of the mouth, tongue, lips, etc., going from low note to high note, the sound will tend to get nastier. Keeping the same shapes going from high to low will tend to dull the sound. So, we modify the shapes to make the best tone for each note. We should also aim for the best vowel for each note. Change how we make the vowel for each pitch.
I know that the vowels tend to go away at some point above the treble clef, but Steber showed that “Now is the night one blue dew” can be reasonably approximated up to at least B flat. Was it the same shape and or feeling of her “oo” in mid-range? Of course not. But, it gave the impression of “oo” all the same.
So, go ahead and modify, but don’t modify the vowel, modify how you make the vowel.