I have noticed that my choirs often sing less-than-pure vowels when the end of the syllable/word contains a singable consonant. R's, L's, and N's are especially troublesome. I call these polluted vowels. Vowels can also be polluted when anticipating the second half of a diphthong.
Imagine you have a bottle of drinking water. Now add just a bit of ink. It isn't like you dropped a dime into the bottle, where you can see it shouldn't be there but can drink the water anyway. The ink has polluted the water throughout, turning what was once clear into a murky grey. That's what singable consonants/diphthongs can do to a vowel - tainting the purity to our ears.
My Men's Chorus is singing Laudamus, a Welsh hymn, done in English. Many phrases end with the word EVERMORE. When they hold the last note, I can so clearly hear the R polluting the OH vowel.
Just say the word MORE in slow motion. You'll feel the point at which the OH has been compromised, but it comes before you are sustaining a pure R. Try the word bound, and notice the collapsed point between the AW and OOH of the diphthong. These are polluted vowels, and choirs sing them all the time because they cling their speaking constructs of the language.
Fear not-I have a solution! It's called word switching. Let's try it on this MORE scenario. I had my men sing and hold the chord on MORE. Polluted. I explained the concept of polluted vowels and asked them to only sing a pure MOH with no R. Didn't work. Then I asked them to sing the word MOTION, which does not have a singable consonant following the MOH. "Sing MO-TION, but hold the MOH until I cue you to go on to TION." BOOM! A crystal clear OH vowel!
It works for dipthongs, too. Having trouble with BOUND? Hold the first half of BOSTON. MAKE? Try MEXICO. LIGHT? Try LOBSTER. Get the pattern?
By changing the English word, we are using our singers' tendencies against them. It's a simple trick that can be used in the short term until singing the correct pure vowel becomes a long-term habit. Listen in your next rehearsal and clean up those vowels. No one likes pollution.