So, away from tuning systems for a bit!
If you read Mike O'Neil's excellent guest post, you'll notice he talked about vocal production and vowels as barbershoppers are taught to work on both tuning and blend.
The basic principals are no different, no matter what the style and level of choir you conduct: to get the best intonation, vowels must be matched.
The reasons are again, scientific in nature. Given exactly the same pitch, one can tell the difference in timbre between an oboe, flute, violin, or soprano voice. The reason is that the unique way each instrument (or voice) creates sounds emphasizes different harmonics (remember them from the intonation discussion?), which we now call "partials" (if you love the science, wikipedia has a reasonable article on partials). Above whatever fundamental pitch is played or sung, the long series of partials (not normally heard as separate pitches) are each stronger or weaker, depending on the instrument or voice. There will be a similar pattern for each instrument, no matter what the fundamental pitch.
For singers, each vowel will also emphasize different partials (because of the different shapes we create in our resonanting chamber), creating the unique sound of an ah, oh, or oo.
On a practical level, then, if our singers closely match vowels, the partials (or harmonics) will be emphasized in exactly the same way, leading towards better pitch matching (as well as "blend"). And the opposite is true if our singers use different varieties of vowels that don't match very well--the differing partials emphasized will tend to "fight" and not agree.
There are some ways I begin to deal with this in any choir, no matter young or old, inexperienced or expert. But for a less experienced choir there are some exercises to help them understand how this works.
To demonstrate how vowel affects pitch I'll have four or five singers (on the same part) come in front of the choir, give them a pitch and ask for an "ah" vowel (no prior instruction). After they sing it, I'll ask one of the singers (quietly, so no one else hears) to sing the "ah" as an "uh" -- then they sing it again. I'll then ask them to sing as well in tune with each other as they can. It's rarely a beautiful unison. Then I ask the singer with the "uh" to gradually change to an "ah" and see how well in starts to tune. Finally, I'll ask all to carefully listen to the vowel they sing and unify it and the pitch as much as they can. This is all so they understand in a concrete way (not the scientific explanation above) how much easier it is to tune if they match vowels. I then extend this to the whole group and play with having them sing with a variety of vowels, then start to coalesce on a single, unified vowel. This may take demonstration from me of the vowel I want as a "target" vowel.
This assumes, of course, that you've been working (usually in vocalises) on different vowels and how to sing them already, or are about to.
Another exercise with vowels is designed to get even an unsophisticated choir to hear and understand terminology you will use, in this case the concept of "bright" and "dark" vowels. I think I stole this from Royal Stanton's conducting book, The Dynamic Choral Conductor, which is now out of print. I'll have the whole group sing a single pitch (octaves in a mixed choir) on an "ah" vowel. Then I'll ask them to sing it as if they were a country western singer (with the right age group, Willie Nelson will always bring a certain tone quality!). Then as if they were an operatic basso. Then as if they were a children's choir (you can use your creativity to come up with other examples or analogies). I then show (with my hands far apart) a continuum from bright (Willie Nelson) to dark (operatic basso) and ask them to go back and forth. Next, I ask one side of the choir to sing with a dark "ah" vowel and the other side to sing a bright one. As I bring my hands together, they're to gradually move to the center until they all find a mid-point with the same vowel that has both characteristics in it, but where they sing the same quality of vowel. I then might do the same thing again, but asking them to be very aware of how easily they agree or disagree on pitch as the move from vastly different colors to one that is unified.
This is simple, but allows any group to have a concrete understanding (not theoretical) of what you mean when you ask for a brighter or darker vowel. It also has the effect of letting individuals realize (without pointing it out directly) that they naturally may sing farther to the bright or dark side of the vowel spectrum and have to be aware of that (I will probably ask them to think if they tend towards Willie or the operatic basso!).
Assuming the group understands this, the task is now to get them to consistently work for unified vowels as we sing. This takes repetition and reminders, even with an experienced choir. More about that next time and (if I have time and space) to speak of vocal production itself.