The Russians gave the revolt guns and bombs, without which, they would’ve lost.
Bottom line is that the Russians and Chinese funded and armed Commie rebellions all over the world. Most of them failed. If you do not have the objective conditions for a revolution, it’s going to fail. You have to have a really lousy rightwing regime in power for a Commie revolution to succeed. Anywhere there is a halfway decent or progressive regime in place that treats people well, you never get a successful Left revolution.
Bottom line is the Viet Cong won because they always had mass support. They had the support of the majority from the very start until the very end. In particular, they had mass support in the countryside. Hardly anyone in the countryside supported the landlords. The VC had less support in the cities, but even there, none of the South Vietnamese regimes had much support from the people.
Also the South Vietnamese regimes were seen as puppets of the US. The US was seen as a colonist who only replaced the French. So the South Vietnamese regimes were seen as puppets of the US colonists. The South Vietnamese regimes were always for the rich and against everyone else. Mostly they were for the rich feudal landlords in the countryside, and had no interest in helping anyone else.
The ARVN did not fight very well, probably because their heart was not in it. Further, the ARVN was completely infiltrated by the Viet Cong. For that matter, the South Vietnamese state was completely infiltrated by the VC also.
Commie revolutions failed everywhere they did not have mass support no matter how much money and guns they had from outside. And if the objective conditions were not correct (a crappy rightwing government), then no revolution usually got started, or if it did, it never went anywhere.
Russians and Chinese only funded and armed rebellions that were already underway anyways. These revolts only succeeded where they had mass support of the majority, and the Western client/puppet regime had almost no support.