My own feeling is that in some way the Vietam war much exhausted the expansionist zeal of Communism, after paying such a large price for what they gained, while becoming more well-known for its repression, and need to gain allegiance by force. Had they taken Vietnam quickly i think it would have been encouraged expanded much more.
Meanwhile, the ideological revolution of rebellion against moral authority (with relative little valid quest for a better reality, but which instead sought fantasy thru drugs) which sympathized with Communism, and helped them win over S. Vietnam, also enabled a ideological Communist victory in the US.