Hey, McGrath, the image isn't half bad. Many persuasive reasons are given above to hold vowels. And yet, rules are made to be broken, and if followed slavishly they produce dumb results. My favorite approach is, (1) find a version of the song you really like; (2) work on singing it exactly the way the singer did it (don't worry about imitation, you'll never manage to imitate the original singer so anyone could tell), and (3) when you get to the point where you can think of ways to sing it so it sounds better (if ever), then make the changes. That way, you need not worry about rules or principles.