A common myth one might hear in the vocal world is that diction will solve vocal problems, that one just needs to enunciate.
As with the breath, diction is good but does not in and of itself fix muscular imbalances within the larynx. So, this cannot be the single key to good vocal technique. Further, if the laryngeal musculature is malfunctioning, then proper diction is impossible to accomplish.
Now, a teacher can utilize certain consonant and vowel combinations in exercises to repair and reeducate vocal muscles. Diction within functional vocal training is not a key but can be a guide. For example, if one is instructed to sing a short scale on a particular vowel and consonant, and the person falls behind in the rhythm with the consonant and vowel being distorted, then it is likely that the person is singing too heavy/weighty with excess volume. Hence, too much cord vibrating for those pitches. As the volume/weight is able to be reduced through registrational principles, then one will notice that one is able to stay in rhythm with consonants and vowels being less and less distorted.
Good diction becomes both a symptom then of better function as well as being made possible by that better function.