Good vocal exercises have purpose, and should be designed to meet the growth needs of the student. Even a “warm-up” exercise must be tailored to the student. The reason this is so important is that an exercise can be beneficial or harmful depending on how it is constructed and applied.
For example, maybe a student needs to focus on a staccato pattern using an [oo] vowel in the faletto so as to address an issue with pitch regulation. Now, if the teacher were to instead focus on loud legato using an [ah] vowel in the bottom of the range, then this would not be the appropriate exercise given what the student needs at the moment.
Vocal exercises consist of pitch, volume, and vowel. These three exercise ingredients cause the vocal muscles to respond in certain ways. If these ingredients are combined so as to address the vocal need, then the muscles will respond reflexively, and healthy results are achieved!