In general I am a fan of the “pure vowel”. All of the basic vowels of most Western languages are intelligible up to F5, but from about F#5 and higher, /i/ and /u/ start to become less specific, followed by the others at even higher pitches. I’m not a big proponent of “vowel modification” as a rule, but there are times, sometimes even below F5, when the singer needs to open his mouth in a way that may cause a vowel to change. Singing high and loud, for example, may cause an /i/ (“ee” as in “feet”) to drift toward /I/ (“i” as in “fit”), et cetera.
There are cases in many songs where singers can “let go of the vowel”. The two big ones are upon repetition of a phrase at a higher pitch (as in many choruses), and at the end of a song. In the great majority of cases, a song that ends with high notes does so with a word or phrase that has already been heard at a lower pitch, usually several times. In these cases: emotion first, intelligibility second. The audience will understand the words upon which the last repetition was based. This frees up the singer considerably and can allow the song to go to a stronger place emotionally. Open vowel, open heart!