My dad was shocked to learn that the communists won

Even that is distorted - the Vietnamese people won under incredible hardship. Simple farmers, hundreds of thousands simply refused to bow to imperialist rule. They were helped, but they did the fighting.

But you can also argue that the US did "win" the war or at least their objectives. The objective was to stop the domino effect of socialism and they absolutely brutally devastated Vietnam for many decades. And that was their objective, to punish anyone who dares to redistribute wealth.