[thanhhaimai]

I wouldn't say the Vietnamese alphabet is "transliteration". Vietnamese is one of the most, if not the most tonal language in the world. The same word, speaking with different tones will convey different meanings.

https://en.wikipedia.org/wiki/Tone_(linguistics)

The modern Vietnamese alphabet was developed in 17th century (so it's not a transliteration) with tonal marks as a core feature. The writing language is very phonetic. Within a region with similar accent, if you hear a word, you can write it. And if you see a word, you can pronounce it.

The tonal marks are very important to the language. It allows for rich poetic rules that makes Vietnamese poem fun and musical to read:

https://en.wikipedia.org/wiki/L%E1%BB%A5c_b%C3%A1t

[tsimionescu]

Yes, I had never looked into this and had assumed Vietnamese uses a Chinese-inspired writing system natively, like other languages in the region. Knowing that this is the only writing system immediately made sense of why this is necessary.

[hashmush]

Ehm, like in Vietnam's neighbors Laos (ພາສາລາວ) and Cambodia (ខ្មែរ)? Sure Vietnamese used to (a long time ago) be written in its own version of the Chinese script, I'll give you that. But most languages in the region do not use a script derived from Chinese.