I found this exercise while looking for some audition repetoire. The site was wonderful for audition material. Unfortunately they decided to offer some free vocal training. Here is one of the exercises for proper vowel formation.
Exercise #1: “Ah”
The vowel sound on which all others are built is “ah”. To put your mouth in the proper position, drop your jaw open until you can fit three fingers vertically between your top & bottom teeth. Do not spread your mouth horizontally, or your sound will be flat and sound nasal. Practice holding a comfortably mid-range note on “ah”. Massage your cheeks and jaw while doing so to eliminate tension.
Aren’t you glad we have found a better way?