The simplicity of Hangul is partly because of the simplicity in the Korean syllables. It will be much harder to derive a simple writing system for a complex-sounding language like Vietnamese or English.

Btw, for the 3 countries, Vietnam, Korea, Japan, that were influenced by Chinese writing deeply, each of them has a different approach.

Korean came out with its Hangul writing system. Japanese came out with hiragana and katakana (together with Kanji). Vietnamese came out with Chu Nom ( http://en.wikipedia.org/wiki/Chu_nom) , a logographic script that is even more complex than Chinese characters.