In choral/classical singing, vowel shapes are essential because that is what gives choral singing its unique tone. Without proper vowel shapes, the tone would loose its classical feel to it. Vowel shapes must be tall. To do this, you must drop your jaw towards the ground and form the shape of the needed vowel. You must breath in on that vowel before your sing. As you sing, try to not let the vowel go wide or it will create a different tonal sound. To practice this technique, my teacher would always tell us to disregard the consonants and only sing the vowels. She did this because consonants would cause your mouth to go wide and create that undesired sound. After we got the vowels down, then we could add back in the consonants. Just practice singing only vowels, and it will help you sing more classically.