Singers work to equalize vowel sounds through modifications of tongue, jaw and lip placement. The brighter vowels such as "ee" "ay" and the short "a" as in "cat" are cases in point. These sounds can be modified to match the more muted vowels such as "ih" and "oh" and "ah" and "eh" and "uh." The "ee" vowel can be modified by dropping the tongue slightly to the short "i" position as in "it" and drawing in the the lips from the sides (fish lips). One of the biggest problems in equalizing the vowels in a phrase is the spread "ee" sound. Better to use the modification. It is usually in the upper registers that the vowels need modification the most. Singers often use the trick of singing the bright vowel in the shape of the muted vowel nearest it in tongue position. For example, the "ee" as in "bee" goes to "ih" as in "it." "Ay" as in "play" goes to "eh" (without the diphthong "ee" vanish). The short "a" sound as in "cat" is a special case. In its purest form, it can be sung well and sound beautifully equalized. The problem is that in certain ranges of the voice, the placement is cruelly difficult to maintain properly and the purity is lost making that vowel sound uncomfortable and strained and too bright or mutated.
Consider the traps in the following lyrics:
Brightly beams our Father's mercy
From his lighthouse evermore,
But to us he gives the keeping
Of the lights along the shore.
In the first line, the there are three "ee" vowels, two of which are on unaccented syllables. Then there are three uses of "r" which can swallow the vowel placement into the throat. The diphthong in "bright" and the "ah" in "Father's" can be placed too far back in the throat to match the forward brightness of the "ee" vowels. Equalizing the vowels in just this first line can be a huge task for a singer, let alone an entire choir!
When working with a choir, the director will want to instruct the singers to listen and make adjustments and modifications to help the sound of the choir as a whole. These little adjustments help the overall intonation of the group. They will sound better in tune, better together, and their message will come through more beautifully. Remember to equalize those vowels!