Actually the cultures of north, central, and south Vietnam are quite different. The accents vary so much that people from one section of the country struggle to understand people from another. They have been ruled by different rulers for much of the last thousand years.
Speak to many southern Vietnamese even today and they'll tell you the war was an illegitimate land grab of the more fertile and valuable land in the south by communists in the north wrapping themselves in nationalist ideology.
South Vietnamese ex-pats are not an especially unbiased source. (Consider the Cuban ex pat population, which again tends to be descended from people who backed the anti Castro forces. Not saying Cuba or Vietnam are perfect, but the people who flee a regime aren't the best people to assess it. Our strategy in Iraq was informed by information from Iraqi ex pats.) North, South, and Central Vietnam are traditionally distinct regions, yes, but the war wasn't between the north and south, it's just that the south started out occupied.
I've discussed this subject with Vietnamese, in Vietnamese, all over the country, including Hanoi. Anti-communist sentiment runs strong everywhere. People are smart enough to understand how much the communists have held back the development of their country.
Speak to many southern Vietnamese even today and they'll tell you the war was an illegitimate land grab of the more fertile and valuable land in the south by communists in the north wrapping themselves in nationalist ideology.