Have you ever mimicked someone's accent or perhaps made up nonsense "words" that were meant to sound like a specific language? Kind of like the Muppet's Swedish Chef? I was once with a group of French people who were making fun of the way Americans speak. As they were mimicking us their faces went kind of slack and their jaws and lips barely moved as they said something like "bluh, bluh, bi, bloo, bluh." What they were getting at, I think, was that Americans facial muscles tend to be pretty relaxed when we're speaking comfortably, and often our vowels can sort of morph into very similar sounds. On the other hand, when I first lived in France and was speaking French almost all the time my face hurt. I was using facial muscles more often in ways that I wasn't used to.
It makes sense that American English would sound garbled to a foreigner; the International Phonetic Alphabet has 30 different symbols for vowels, but Americans use at most 16 of these, and most regions use fewer. British English speakers, on the other hand, use about 20 vowels compared to our 16. So, yes, when we speak it may kind of all blend together.
But wait . . . don't singers want to blend? Don't we want to develop that creamy, gooey, luscious legato vocal quality that just makes the sound luxurious? The answer, at least for classical singers, is yes, we do. But at the same time, we want to be heard, understood, and we want build up vocal stamina in a healthy lasting way. This means, then, that the key to creating a smooth silky singing voice is in steady constant air flow (see this post) and vowel equalization.
First I want to talk about vocal placement. Placement isn't strictly about vowels, but understanding vocal placement will be helpful later in the post. I like to use a scale to represent forward and backward vocal placement. If you've ever heard about singing in the "mask" or anything about forward and backward singing, this is probably what it was referring to. I find the easiest way to explain it is that forward placement sounds like the Wicked Witch--"I'll get you my pretty!" When speaking or singing this way you'll probably get some sympathetic vibrations in your nose and cheek bones. Backward placement sounds like Santa Claus--"Ho, ho, ho! Merry Christmas!" This may feel like you've got a frog in your throat. Neither of these vocal placements are correct--they are simply the extremes. Correct placement is in the middle somewhere. Once you've established these two extremes (I'd recommend just practicing with your speaking voice first) you can play around with the degrees of forward and back, basically every placement in between.
There are more than 5 possible placements with micro-degrees of change, but 5 is a good amount to figure out and master when you're playing around--and it lines up well with the vowel placements I want to talk about.
Now on to the vowels. There are more than the five or six that I want to discuss, but these will do for today. In the International Phonetic Alphabet [a] = "ah" as in hot, [ɔ] is "oh" sound as in chores, [ɛ] is "eh" as in bed, [e] is "ay" as in way, [u] is "oo" as in shoe, and [i] is "ee" as in tree.
I've said before that I was a really hard student to work with, right? Because I needed things explained so many different ways before they clicked? So, the following explanation is really the marriage of things I learned from several different teachers.
So, how can you fix this? First, you can think of sending your vowel in the opposite direction than the way that they "want" to go (send your [u] forward towards a 1, and your [i] back towards a 5). Ideally all of the vowels then end up in the same place in the middle--I personally "feel" that this placement for me is between my molars. All of my vowels should "land" there. Another way to get all of your vowels into the same placement is by singing vocal exercises that switch quickly between two vowels; this will help bring the two vowels to the same placement, thus "equalizing" them. There are some vocalizes in the materials section of my website that you try if you want (here).
This chart gives a more detailed explanation of how the vowels should be formed. notice that the vowel "opposites" of [i] and [u] use only the lips to change back and forth between the two. The jaw and tongue shouldn't move. They don't need to move. Try it and see. When you're moving smoothly between the two you might feel like you're saying "wee." [o] and [e] are the same, only lips involved. They become "way." Now, if you then want to equalize [u] and [o] for example, the lips stay in the same place and only the jaw needs to move. This then becomes "woah" when sung smoothly. [i] to [e] becomes "yay."
If you can master this sort of vowel equalization you will have mastered the most efficient way to create legato. You will have also eliminated any tension that may come from vowel formation and increased your vocal endurance. It takes practice, but it's definitely worth your time.