,This is my very simple, no nonsense, rule to vowels when singing.
The ones crossed out in the picture are NAUGHTY VOWELS SOUNDS. They make the acoustic space inside your mouth detrimental to singing well: they raise the tongue root, push from the throat, tighten the jaw. They are very, very NAUGHTY.
The ones of the right side of the list are GOOD VOWELS SOUNDSand conducive to singing well. They encourage the top of your mouth (soft palate) to lift, they don't encourage tongue tension or pushing, they open up the mouth (pharynx) and throat, and loosen the jaw.
IMPORTANT NOTE: When I say vowels, I mean phonetic vowels, the sound of them not the vowels as you learn in school.
Exercise: Sing your songs through, circling the NAUGHTY VOWEL SOUNDS and make them GOOD VOWEL SOUNDS.
Tip: Sing the naughty vowels and drop your jaw. It will usually transform them into good vowels.
Let me know how you get on or if you have any questions.
Any smarty pants out there who note that some vowels are diphthongs (double vowel sounds). Yes, you're right. For more on diphthongs, take a read here.