Geographically it was half the country, but population wise it was closer to 2/3rd pro communist vs 1/3rd anti communist. US involvement wasn't really justified to start and mostly sunk cost fallacy with how they tried to support the French rule before France pulled out and US was holding the bag and a doctrinal choice of stopping the spread of communism even when there was little to nothing to gain and only save face. Vietnam was going to have a civil war no matter what but US definitely made it worse and drew out the conflict and ramped up the death toll with nothing to show for it. If the US had any intentions of taking advantage of Vietnam modernising and industrializing they'd have setup southern cities that were more friendly to US investments with trafe and infrastructure. But just like in Korea that wouldn't happen for decades later, US presence there was entirely military and some very bare bones humanitarian aid.