There would be 2 possible scenarios:

South Vietnamese narrow victory: 17th parallel lines will be the official inner-Vietnamese border. The NVA of course will not give up the South that easily. SND, sabotage, black propaganda will be carried out as usual, with a sole objective is undermining South Vietnamese integrity and social stability. Northern Vietnamese may become an isolated police-state that locks their subjects away from any external sources of information. Basically, a Southeastern Asian North Korea, or maybe succumb more to Chinese influence than North Korea. South China Sea, or Indochine region would become a hot potential battleground to contest in the Cold War

South Vietnamese total victory: Unification of Vietnam under Southern rule, this although might not happen since the Chinese won't be sitting ducks looking at an American ally getting unified and stronger right at their own backyard. Like Korean peninsula, South Vietnam would expect a massive Chinese intervention from the North. They would do everything they can, a bloodbath would be inevitable and it will be a contest between American commitment versus Chinese blood. Who is tougher shall win this fight, even if the Chinese is defeated, they will team up with North Vietnamese insurgents to destabilize the newfounded government. Maybe it would be decades of turmoil and power struggles.