> No dominate written language at the time of European Colonialization

Vietnamese used to be written using Chinese orthography just like Japanese.

The French forcibly cracked down on this form of orthography, and following independence, later modernists attempting to copy Ataturk along with latent Sinophobia due to the Chinese colonial era meant this for of orthography has largely been relegated to ceremonial usage.

A similar thing happened with Bahasa Indonesia, as Indonesia's founding leadership was more secular and socialist in mindset compared to neighboring Malaysia where Jawi remained prominent because of the Islamist movement's role in Malaysian independence.

Another factor is that literacy rates were very low before colonization, in Vietnam to read or write using Chinese characters was never a broadly known skill (outside of the elite). This is a pretty big contrast to Japan, which had double-digit rates of literacy during the same era.