To put it more broadly, when South Vietnam was defeated/unified, what was done to "make it socialist"?
The new communist regime had several levers by which to transform the formally capitalist society in South Vietnam. It first used Reeducation Camps to indoctrinate the former members of the South Vietnamese bureaucracy, military, religious officials, and intelligentsia in socialist propaganda. Ideally, these places would be where the former regime members would learn how to integrate into a socialist society. However, these camps also doubled as hard labor camps. Hundreds of thousands were sent, told they would be going for days, weeks, and months (for the highest level officials). The last South Vietnamese generals were released in the 1990s. Thousands died in the camps, often buried in mass graves.
The next method were the "New Economic Zones." As a result of the mass exodus to Saigon in 1975, the population of the city and its surrounding towns and villages was unsustainable. Millions of people lived in and around Saigon, after the evacuations from the Central Highlands. While the overall goal was land reclaimation, the ulterior motive was to redistribute land and property from "bourgeois" southern Vietnamese landowners and capitalists. In many cases, shops, storefronts, and homes were seized and entire families uprooted, directed to move to the New Economic Zones.
The zones themselves were often remote sites in the mountains or jungle, far away from South Vietnam's urban areas. Southern Vietnamese would be forced to relocate, leave their property and belongings, and work the land. There was often no real infrastructure in these areas, and many died of disease or starvation. Unlike the Reeducation Camps, it was a little easier to escape these remote zones.
While thousands of Vietnamese were relocated, many thousands of Northerners would move to the south. The idea behind the New Economic Zones was essentially land redistribution at its core. Many of the people who suffered under this program were ethnically Chinese (the Hoa people), who often owned businesses and land in the south. Already discriminated against in general by ethnically Vietnamese people, the Hoa people suffered greatly at the hands of the communist regime, as they represented capitalism at its apex.
To top it off, with the seizure of southern farm lands and its forced collectivization, people were required to contribute their crops to feed the army, and what little was left was rationed to the populace. In addition to the widespread inflation and economic turmoil, this forced collectivization and central planning of agriculture contributed to the widespread famine that wracked Vietnam in the late 1970's and throughout the 1980's, eventually leading to the communist regime's acknowledgement of failure and eventually led to the Doi Moi (Renovation) policy, Vietnam's adoption of market-oriented reforms—the Vietnam of today.
Sources: Harish Chandola. "New Economic Zones." Economic and Political Weekly 12, no. 4 (1977): 83-85.
Chapman, William. “Hanoi Rebuts Refugees on 'Economic Zones'.” The Washington Post, WP Company, 17 Aug. 1979.
Repression in the Socialist Republic of Vietnam: Executions and Population Relocation by Jacqueline Desbarats
Brian Van Arkadie and Raymond Mallon, VIET NAM: a transition tiger. Asia Pacific Press, January 2004