Why couldn't beat Vietnam?

Do you mean North Vietnam? Because we've never fought the country of Vietnam. North Vietnam we fought and beat, forcing them to sign a peace treaty with the South. We didn't completely debilitate their capacity to wage war, which we probably should have done, but we achieved our political ends. Then the North violated the peace accords and the US Congress abandoned South Vietnam so the communists took over ... but the portion of the war we fought, we won.