Bonney [a] vowel tends to take on a myriad of shapes (especially through her melissmas) but somehow has the ability to maintain a consistent vowel sound. I'm not sure how she pulls this technique off, but it works for her. Perhaps vowels modification is something that all singers must play around with. I also notice that during her tight melissmas (notes floating around no more than an interval of a perfect fourth) her mouth is more closed singing on the [a] vowel, however, when there is a sudden leap in the music directing her to sing higher, her mouth widens more and more to give her more space to the upper register. Commonly as singers, when we think high in register, we psychologically believe we should squeeze, and make a smaller space, when in fact, it's the complete opposite of what should be exercised.
Polenzani has a really weird but gorgeous [a] vowel sound. On the high corta sung in the early stages of the aria sounded like a mixture of a pure [a] vowel sound and [u] as in "uncle". This seems to really work wonderfully for him. I believe that it was a slight cover that possibly made the [u] vowel present on the high note. Again, like Bonney, the higher he sings in the register, the wider his mouth becomes. Now this is either a really good technical concept approach to singing healthy in the upper parts of the voice, or this very well may be just one big coincidence. I'll take my bets on it being a staple of having a good technical approach to singing in the upper registry.
Good [A] Vowels from Nicolai Ghiaurov; Click here to view