I am unsure of this statement.
The Vietnamese see themselves as one people, but they are divided by a border.
It isn't the border that defines them.
There are many such groups that share a common heritage and history which have nothing to do with the lines currently drawn.
The border defines who's tax cattle you are.
It is usually a meaningless line on a map.
Take the border between Alaska and Canada, without a GPS there would be no way to tell where that line was at all. And even if they actually carved a line there, it wouldn't make any difference to anyone walking out there.