We didn't even have to win to win. If the US was willing to pay the price Vietnam would be partitioned to this day. There's no way they could have defeated us militarily even if we didn't invade the North. But the public wasn't willing to pay the price. I suspect we wouldn't even have needed troops on the ground - the South Vietnamese destroyed a large invasion force with the help of US air power in 1973 and might have been able to hold indefinitely if we had continued to provide air support.
It sucked for the people in the South, but I wonder if the country isn't better off today. Like China, Vietnam only pays the barest lip service to communism these days. They have more economic freedom than Americans. If we had forced a partition maybe Vietnam would be like Korea, with both armies on a hair trigger and Ho Jon Un building nuclear weapons to intimidate the neighbors.