Why Chinese is hard to learn.

Vietnamese has accent marks --diacritic-- for that.

It's not like in french where it changes the vowel entirely (e.g: e as uh -> é as eh -> è as err), but like you described.

Yeah, in french accent marks are either:
Phonetical: é, è, ê, ï, ü mainly
Differential: two words pronounced and written the same except for the accent mark (a and à, du and dû and many others).
Inherited by origin: generally the circunflex accent where there used to be an S (like hôpital)

In portuguese:
tonicity marking: to mark which syllabe is pronounced stronger.
differential: same as in french.
nasality: just for ã (nasalated A)

¨ has just been taken out of the portuguese language, it was used to differentiate the pronounciation of a U after a Q. Now we are left to guess how to pronounce correctly.