I wrote this previous comment about the Tay Son Uprising that does address your question within it. Basically, the Mekong Delta really became Vietnamese as the Nguyen lords expanded south during the later Le Dynasty.

I also want to point out here that culturally, there actually is a bit of a difference between central and south and the north (which is the center of Vietnamese culture). The central and south was fairly multiethnic for a long time, with lots of intermarriage and cultural mixing, right down to religion (though that is more contested). Even the Tay Son Brothers of the aforementioned rebellion were quite possibly ethnically Cham in background rather than just Vietnamese. The central and southern regions became “melting pots” of sorts. Nowadays even the food cultures and things like that have diverged from the north some.