Perhaps revolution run out of countries...
Frankly, I do not think that the lack of revolutionary movements after Vietnam had anything to do with the US. Revolutions usually happen in rural countries of certain profile: semi-feudal systems, a lot of absentee landlords, weak urban working classes. Such countries are in an increasingly shorter supply, hence no fertile ground for revolutions.
In fact, not a single country that has even a remote claim to modernity and democracy was taken by a revolution. Fascist coup - yes, but not a communist/leftist revolution. There are good explanations of that, from Gramsci to Barrington Moore and to Rueschemeyer et al. - all boiling down to the observation that revolutions are mainly a feudal phenomenon.
Wojtek