Aa Ăă Ââ Ee Êê Ii Oo Ôô Ơơ Uu Ưư Yy
On each of which you can add 5 different tone marks: https://vietnamesetypography.com/alphabet/
Plus there are diphthongs and triphthongs (look at the zero coda and off glide coda columns): https://en.wikipedia.org/wiki/Vietnamese_phonology#ref_Notes
Disclaimer: I know stuff all about Tiếng Việt, but it seemed really hard to learn how to hear and say the sounds when I tried for a few days with private lessons. Using so many diacritics modifies the Latin alphabet so much that it becomes something else...
The Khmer alphabet seems to have 33 consonant letters, 15 vowels, 16 "dependent vowels" (which look to attach like an accent to a consonant), and 12 diacritics.
Since I don't have any knowledge of either language, I don't know if this is better.