Keep in mind...
This is NOT about how great singers sing (and phrase a song). That's a different discussion.
It's about how a song is WRITTEN using what will be "vocal rhythm" to get a lyric across.
Exactly. How does a person write even a second line (much less a second verse) if he/she doesn't know how it's going to be sung--the rhythm of the words. And it may include syllable count, but it's not limited to that. Inflection, strong and weak accents. How much time does it take to deliver that line or phrase or word--more-or-less a beat. a bar, two bars, three and a half bars? As many other considerations as can be brought to bear. And most importantly, WHY? What is about the lyric as a whole and a phrase or line in particular that merits/begs for/requires these considerations?
Take "bubble". Why does BUUUUbble sound natural in a song, but BubbLLLLe sound wonky and awkward? Neither is the way anybody says it. Those choices are worked out in the write. That's an extreme example, but I guarantee you someone has written a song and sang "bubbLLLLe" to try to match a melody. I see that lazy so-and-so in the mirror every day.
Tempo and key can be changed. Melody is malleable and is itself influenced by the rhythm of the words. The "band" can take a groove in many different directions. But the rhythm of the words in a song is what the song "is" at it's core.
I contend that if a writer doesn't make those choices in the course of writing (is unable mentally or physically to "sing" those words), then he/she hasn't written a song at all. It's a poem and wannabe song-lyric. If he/she can but his/her choices have simply been sub-par, then at least it qualifies as a song.