Turns out that, like so many of the ideas pooh-poohed by the vacuous (or complicit) Left during the Woodstock days, this one was far more true than false. When Vietnam fell, Cambodia and Laos followed shortly, with a harvest in blood unseen since Stalin's purges.
Even in the countries that never went Communist - Thailand, Malaysia and Indonesia, the mainstream have become very soft toward Communism, and other procrustean ideologies, including Islamism, in far greater measure than was the case during the 1950s. The US failure to hold our ground in SE Asia (including the disintegration of SEATO) has led to a geopolitical power vacuum. The remaining free nations of SE Asia have increasingly been kowtowing to Beijing and are no longer necessarily reliable allies of the US. While not completely falling to Communism, they have certainly gotten into something that resembles Finlandization.