I don't know how many people write songs based on which vowels to choose and how to use them in a bridge....???
I think there would be hardly anyone writing a song based on which vowels to use in a bridge. There's no suggestion that the OP is seeking out particular vowels for musical reasons without considering their lyrical context.
Personally, in the process of writing songs, I've had a number of equally acceptable lyrical choices for a line. I've made the choice based on what I believe is the most musical. So I may be making the choice based on vowels, but that choice doesn't negate the lyrical content.
Not saying it's out of the question and/or not possible...just a rather strange way to write songs.
No, it's not out of the question, and yes, it is strange. But there are plenty of examples where this has been done: "Oo ee oo ah ah ting tang walla walla bing bang" from "Witch Doctor" is a good example.
Most Rock/Pop songs start with a groove and some basic idea of the story, the lyric...
and you get to a point with the lyrics where it's not an endless selection of words and/or vowels, but a very finite selection...once you start writing them. IOW...after the first couple of lines, your goal would be (I assume) to stay true to the "story", and there are so many things that you can say, so many words that you can use...that will fit that story, and work with the groove.
I don't agree with this. I think this is a self-limiting assumption. I agree that you can sometimes 'write yourself into a corner'. This happens when you write a marvelous line, then struggle to find something to rhyme with it. If you persist with the original line, you get nowhere. So you have to change the first line and re-craft the thought differently. The 'finite selection' is imaginary, and choices are limited only by your imagination.
Of course, it it's mindless lyrics and sophomoric rhyme...then guess any choice is possible just to make it work.
It's usually the sophomoric and mindless lyrics that reveal lack of musical and linguistic knowledge. It is the writing of people who write without skill or art, just to make things work.
Gifted songwriters know how to use vowels to support the musical context, and they have the vocabulary to access words that do this and support the lyrical content.