Same as Vietnam: No dominate written language at the time of European Colonialization.
I mean I note that there are some Chinese languages, with millions of speakers, where the largest written text they have is a bible written in a Roman script. If those are a challenge surely Vietnamese must be as well.
Not very well. The old vietnamese script with Chinese characters had a lot of custom additions not in Chinese to make it work. It clearly was ducktaped.
The colonial administration didn't have to push too hard to make people switch, the customized chinese script wasn't very popular.
Every now and then it will pop up in the news due to politicians using it as a tool to cause racial divide.
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.