The situation in Vietnam was unique. The United States had won the military side of the war, but in the 1960s there had been a counter-cultural revolution in the United States which had introduced the passive, progressive values that dominate the United States now. The people who bought into this self-loathing defeatist rhetoric were able to change the course of history, just as the Cultural Marxists of the 1920s (who first came up with the ideas that dominate western societies today) had hoped.
Granted we were having a tough time in Vietnam, the negative coverage of the war and protests didn't help. This put immense pressure on congress and caused the making of rash decisions. I think the war would've turned out very differently if there was no media coverage of the war.
One major problem with the war was that the South Vietnamese didn't want it. They mostly wanted to join the North. As the war progressed more and more South Vietnamese saw us as unwelcome interlopers in a civil war, making the pro-capitalist side (I'm NOT saying pro-democracy as the S. Vietnamese regime was NOT a democracy) more and more unpopular.
There are too many dictatorships in the world for us to involve ourselves in overthrowing all of them and quite frankly there are parts of the world that would benefit from a liberal (I'm using it in its global context not its domestic one) dictatorship rather than a democracy. If a democracy will vote for religious oppression, racism, and bigotry then that country is better served by a dictatorship that is against those things. I am talking of course about middle eastern countries. Many are just not ready for democracy. Dictatorships dedicated to liberal values (again by liberal I'm not talking about the domestic liberal v. Conservative contest I mean in terms of how the word is used in international politics) could help create the conditions necessary for a transition to democracy while holding back elements that threaten freedom.