It was our fault, beyond question. We didn't like the fact Ho Chi Minh would have won 90% of the popular vote (which is why we weren't trying to push for democracy), so we set up a puppet government with a corrupt Diem (who we shipped out from New Jersey). Once we tired of him, we assassinated him anyways.
When the Cambodians wouldn't help us invade Vietnam, we overthrew their popular leader, Sinahoek, and bombed more millions there, which left the vacuum for Pol Pot to take over. If it's not our fault, who deserves the blame for carpet bombing all those people in Vietnam, Laos and Cambodia, merely because we didn't like what leaders they elected?
To say it's not our fault is the definition of American arrogance and imperialism. No one here questions that the Soviet invasion of Afghanistan wasn't the Soviet Union's fault. They wanted to control the valuable oil pipeline there. When analyzing US actions, however, many Americans, comfort themselves that we were actually trying to help those people, from having the leaders they wanted to elect, by bombing the shit out of them. If they bombed us because we elected Bush, I'm sure you wouldn't blame them either.