In Vietnamese there are 6 tones and the grave accent is used to indicate the falling tone. "ù" itself means "fat". But the tone is applied to every syllable in Vietnamese, hence any vowels can have the grave accent on them. For example à, ằ, ầ, è, ề, ì, ò, ồ, ờ, ù, ừ, ỳ...
You can also see that in Hanyu Pinyin like yùnmǔ (韻母)
I'd assume it's also common in other tonal languages which use Latin script and diacritics for indicating tone
Some other non-tonal languages that also use ù
In Ligurian, the grave accent marks the accented short vowel of a word in à (sound [a]), è (sound [ɛ]), ì (sound [i]) and ù (sound [y]).
In Scottish Gaelic, it denotes a long vowel, such as cùis [kʰuːʃ] ("subject"), compared with cuir [kʰuɾʲ] ("put"). The use of acute accents to denote the rarer close long vowels, leaving the grave accents for the open long ones, is seen in older texts, but it is no longer allowed according to the new orthographical conventions.