Basic vowels are sung as single sounds, rather than compound sounds. Take the vowel (I) as in the word time for instance. (I) is really a double or compound vowel because you have to say or sing
AH + ee to actually get the (I) sound in it’s completion.
You must actually say or sing in a rapid succession to produce the entire vowel.
Compound vowels are actually called diphthongs . Meaning two sounds.
Understanding how to handle these compound vowels or diphthongs is a necessity which is needed to be understood by all vocalists.
Sometimes diphthongs have constrictive vowels at the end of the 2 sound. Constrictive vowels can cause the mouth and throat to close, just the opposite of what's needed to sing freely.
Whenever possible detach from the compound vowels to allow for more openness in your melody line. The general rule is to sustain on the 1st of the 2 vowels sounds and quickly end on the 2nd vowel sound.
Below is an example of the vowels A, I and O and how they are split into 2 valves when singing them.