Some years before the war started, the factions that became the North (majority) and South (minority) participated in negotiations at Geneva. The deal stipulate

Some years before the war started, the factions that became the North (majority) and South (minority) participated in negotiations at Geneva. The deal stipulated national elections, with interim partition (North and South) after a delay to allow migration. But the South faction, realizing that it would lose, declined to participate. And that's what precipitated the conflict. It's crucial to keep in mind that it was the North faction that had been fighting for independence from France. And that the South Faction comprised largely the colonial bureaucracy and Catholics.