Every song has two melodies. The first is what is to most of us the obvious one: the way the notes move one to the other. The second is more hidden: it is the way the vowels move from one to the other. Once you are aware of the Vowel Chart and most especially the division between the round vowel family (Oo-Uh-Ah) and the split vowels (Ee-Eh-Agh), you will know that each vowels is very absolute and specific. Just as much so as a note on the piano.

Each vowel has a character and mood of its own - again, just like a note on the piano. Moving from one vowel to another thus creates a particular effect. A movement from Ah to Oo is a closing effect. Uh to Eh is increasing aggression and brigtness. It is a bit more complex than this as context can alter content and effect of a vowel, but you probably get the idea.

So perhaps a songwriter uses an opening movement... say Uh to Ah on a rising line. In that case the melodic movement and the Vowel movement complement each other. The writer may however choose to counterbalance a rising line with a closing vowel, to different effect.

As a singer, you need to study and understand what the songwriter has done with the vowels in order to properly deliver the song. Plus of course you need to see how the consonants have been used to enhance the vowels and create rhythm and percussion in the song.

Consider this line from Patsy Cline's "Crazy"

"For thinkin' that my love will hold you." What do we have here? If we look at the key vowel words we have:
Thinking .. My Love.. Hold You, which yields:

"EeEh Ah Ah Uh Oo" See what I mean? A distinct pattern emerges. Analyse some songs: there are some suggestions on the right. Work with some other songs... including the ones you are writing. Are the vowel movements real and meaningful?

Think of the vowel movements as a gift from the songwriter to you!