Such lovely things, vowels, they carry who we are, our vocal identity, our timbre, our ‘money notes’ and yet we often sing them without a thought! Choral singers need to blend and match vowel colours but it is no excuse to sing badly. You can sing with open Italianate vowels in a choir and achieve so much more resonance without tiring out the voice. Yes we sometimes, especially if we are male, have to modify the vowel colours at the top of our range, but do use your ears and think before you sing. Breathe in on the vowel, get used to that vocal space and later on I want to talk about background vowels ( in French song especially).
The Bel Canto singers worked on vowels, it is now rather unfashionable to do so perhaps, but along with crisp, clear consonants, it is our way of communication and it goes back to the primeval grunts and groans of the caveman. Something fundamental is transmitted if you get your vowel on the spot.
It is all to do with the magical resonance of the vocal tract and you must remember that the colours we can create with our singing voice are thousand fold. Use the space inside and outside of you when you sing, think acoustically and get those ears really perceiving the difference between exciting sounds and dull uninteresting ones.