“You can be a dominant power in a region without invading other countries.”
But they weren’t. They weren’t even interested in interacting with these other cultures.
Vietnam and Korea were simply the same people, same culture.
The fact that they became separate actually argues against Chinese dominance in that they couldn’t even keep their people at the peripheries.
You’ve got an upside-down way of looking at one of the most persistent and advanced and expansive cultures through the ages.