In recent years a great deal of attention has centered on formants, or the shaping of the vocal tract for defining and resonating the vowel, and especially with respect to positioning of the articulators and mouth. However, all effective control over the tone production is lost once the vocal folds become vibratile. Consequently, the vocal tract and the articulators are not of primary functional importance and, while a proper selection of various vowel phonemes and the deployment of those moveable parts lying above the vocal folds can be useful on occasion, the extent of their ability to improve events transpiring at the functional core of the mechanism is minimal and should not be overestimated.
Reid, Cornelius L. Essays on the Nature of Singing. Recital Publications, 1992.