Well, they've always only been kind of sort of communist. Half of the "Communism" that the government pursued was just the screwing over of the landed classes that we saw in Taiwan and US-Occupied Japan OTL. It was more nationalism with a socialist and anti-gentry flare than outright Communist. The US base in Cam Ranh Bay and the formation of Special Economic Zones in Cam Ranh, Phu Quoc, and Guangzhouwan (which France ceded to Vietnam) also built the relationship up well.
Truman responding to the Proclamation of Independence of the Democratic Republic of Vietnam by telling the French that they'd be allowed to have Saar, Aosta, and Elba if they gave up Indochina set the stage for the anti-Soviet pro-western Socialist regimes. Vietnam was the first, but its geopolitical example paved the way towards the de facto entry of Yugoslavia into NATO.