That's not what makes a vowel a vowel
If U makes the U sound and not the Uh sound without the assistance of another letter, it's a vowel. Not much to argue there.
For every rule you think you know about English, there are more exceptions than things that follow
rules tend to get you in trouble~
So then H is a vowel? As is "an hour.
Fun, gun, run, spun, etc. words would need more letters