While there are thousands of vowel sounds in the world's languages there are only five important ones for singing in any language:
I, E, A, O, U, which are pronounced eee, ay (as in hay), ah, oh, and oooo (as in pool).
The vowels are not arranged in alphabetical order, but rather, in sound order…
Over the decades I have heard thousands of singers audition, perform, and practice. As a director of choirs and musical shows I followed how singers sounded after trying various singing techniques, vocal methods in books, and expensive coaches. I learned what works, and even more I learned what does not work for singers.
One of my earliest memories about my musical life comes from fifth grade, when I started singing in a choir. We had two one-hour practices a week, in an empty classroom, with only an old piano that our Choir Director, Mr. Garvin, would repeatedly play to teach and remind us of our phrases.