American sphere of influence:
West Germany, capitalist, prosperous
South Korea, capitalist, prosperous
Japan, capitalist, prosperous
Soviet sphere of influence:
East Germany, communist, impoverished
Belarus, communist, impoverished
Vietnam, communist, impoverished
Chinese sphere of influence:
North Korea, communist, impoverished
Even China under communism was impoverished.
This is not a difficult decision to analyze. America promotes capitalism, and it has no territorial designs on Vietnam. The question isn't why Vietnam is making this choice, the question is why it took so long.