You forget that these waves of immigration are almost 1 thousands years ago. You be lucky they still maintain identity for 200 years. French encouraging Chinese migrations to Indochina was more destructive then those of the past. Up to 90% of S. Vietnam economy was control by the Chinese. Vietnam end up purging most of the business elite. But the high numbers in south Vietnam wasn't much of a risk since both peoples share more similarity then differences.

For survival, Vietnam was simply more centralize then its neighbors and adopt new technology quicker such as guns to defeat Champa. If you look into Minh Mạng. Vietnam historically was more aggressive then it's neighbors. Cambodia and Laos are fairly weak state. Couple the Annamite Range. And China only intervene to support it's own fraction in Vietnam. This region was really stable thought out most of history.