How does one know when the tone mark goes on the first vowel or the second vowel? This may have been cover elsewhere so I may have missed it.
In wikipedia they mention a new style and old style. The old style (hóa, hủy), the "new style" (hoá, huỷ)
Don't let it bother you. Honestly, I don't think it matters.
Personally, I preferred the old style, Because that was how I was taught as a kid.
The Vietnamese don't need to make things more complicated.
Oddly enough, it has made its way into the oxford dictionary. At least it came up when i searched it online. Odd that wanna made it but wannu (the previous step in the transition from 'want to' -> 'wanna' did not make it.