Control of the musculature used in singing should be indirect. If we are distracted by direct control, our singing will be boring at best.
We need to control how the musculature works by focusing more on the vowel. We control the vowel through the emotion or attitude we assign it (for example, sad, happy, angry, etc.).
This might sound esoteric, but it's not. We are not completely different from other wind instruments (saxophone, trumpet, etc.) You can ask, for example, any trumpet player how easily he would be able to play his horn if somehow the bell were bent out of shape. He'd tell you, not easily and with no certainty of how it will sound or how consistent the pitches would be. The difference is that the trumpet cannot willingly change it's shape to create different sound qualities on different notes. That's the trumpet player's job and he or she does not have near as many possibilities to colour the sound.
Every vowel feels different on every note we sing so we need to stay focussed on it and make sure we let the vowel change as we move through our range. For example, the "E" that we sing on D above middle C will not feel nor should it be formed the same as the "E" on D above high C.
If our vowels are ill-formed when we sing, we will not have this indirect control that we so need to have freedom of expression.
When I say "ill-formed" I mean when there is no attention to it. Any vowel shape gives its own sound quality and resonance, so we can really play with how we sing our words (using different accents, for example) and have what would be called terrible diction but it's not wrong if you actually mean to produce the vowel shape you're using.
So, I'm not saying that you have to sing with perfect diction. But you have to mean whatever vowel you sing. There's a reason I say this. Many of us use our diction and vowels when we're singing a song, but when we sing through a vocal exercise, sometimes we totally forget about the vowel we're using and instead, concentrate on tone production, or the diaphragm, or whatever else we think needs attention.
So, Ah becomes Uh. It's not that the Uh won't work, but if you sang that when you meant to sing Ah, being distracted by some notion of direct control, you're going to find that you don't get the results you want. Intonation will be difficult, the breaks in your voice will be more difficult to navigate, and you'll sound like you don't know what you're doing.
When we are trying to control some musculature supporting the larynx, for example, we cannot do this directly; we need to create a vowel that will cause the muscles to work in the intended way.