They didn’t. The Viet Minh had some popular support as they were fighting to expel the French. But after the negotiations in Geneva that established a North and South Vietnam, the citizens could move to whichever country they wanted to live in. Over a million Vietnamese moved south. Less than 50,000 moved north.
Since very few of the Vietnamese were enamored with communism, but the communist felt that they knew what was best for everyone, they left a cadre of guerrilla fighters behind when they moved north. These were called Viet Cong.
The Viet Cong were able to recruit some people to fight against a very Trump-like corrupt government that was bent on self-enrichment and not building a vibrant democracy. With logistical support from the north and indirectly from USSR, the south’s future looked uncertain. So the south asked the US for help.
When the tide looked like it was turning against the Viet Cong, they committed their first strategic blunder; they adopted terrorist tactics. Anyone who has read Che Guevara’s or Mao Zedong’s books on guerrilla warfare would know that a guerrilla army without popular support will fail. No Army that terrorizes the populace will be supported by the populace.
The rest, as they say, is history. But the most telling bit of history is that it was not the Viet Cong that eventually conquered South Vietnam but the North Vietnamese Army. After Tet of ‘68, the Viet Cong had become so hated that they could not recruit. So the only effective communist fighting force in the South was the North Vietnamese Army.
One need to read American histories of that war with a high degree of scepticism. The Vietnamese war was extremely unpopular among the American intelligentsia (as it should have been). These same intellectuals wrote the history books often portraying the war in a way that supported their political opinions rather than presenting a balanced view of history.