In Ken Burns' documentary The Vietnam War, former CIA analyst Frank Snepp claims that shredded documents from the U.S. embassy in Saigon were taped back together by the North Vietnamese after the city fell, and the documents were then used as a "blood list" to track down Vietnamese people who had worked for the U.S. government or military. Disregarding whether this story is true in its particulars, what happened to the Vietnamese people who had worked for the U.S. embassy, CIA, MACV, etc., and who couldn't/didn't escape when the South lost the war?
The aftermath of the Vietnam War and of those who worked as South Vietnamese bureaucrats, upper level officials, low level associates of the government, members of the military, and civilians can be divided into several categories.
There are no reliable numbers on how many were summarily executed following the war. We know it happened, and there are significant anecdotal reports, including photos of kangaroo courts, but no real numbers. Colonel Ho Ngoc Can was province chief of Chuong Thien Province, and elected to continue fighting following the South Vietnamese ceasefire. He was captured, and eventually executed in Can Tho stadium in August, 1975. There are purported photos of his imprisonment/trial/execution, but they cannot be confirmed. There were perhaps dozens of cases such as this, but certainly not widescale. Nothing of the "tắm máu" that was rumored, though that is not to say there were not hundreds of thousands who died after the war.
Some of the tribal groups in the Central Highlands elected to continue their guerrilla war following the ceasefire. The largest of these groups, FULRO, would continue to attack police stations, checkpoints, and remote outposts throughout the late 70's and 80's. The last group surrendered in 1992, seeking asylum. They received tacit support from the People's Republic of China.
Some Vietnamese refugees in the United States organized themselves into a guerrilla and terrorist organization, the largest and most prominent called themselves the National United Front for the Liberation of Vietnam, and was headed by Hoang Co Minh, a former South Vietnamese Navy commodore. They extorted money from the Vietnamese-American community in order to fund a training camp and weapons purchases in Thailand. The United States government was aware of their activities, both in the United States and in Southeast Asia. Most of their members (Vietnamese-Americans who had volunteered to fight) either returned home or were killed in border clashes in Vietnam.
The Front would later renounce violence and reform itself into the Việt Tân political party (itself illegal within Vietnam).
With Saigon and its suburbs bursting with refugees and straining the South's resources, the North had to find a way to forcibly relocate people, as well as implement its socialist economic practice onto the South's economy. The idea was to redistribute land and property from southern Vietnamese landowners and business owners. In many cases, shops, storefronts, and homes were seized and entire families uprooted, directed to move to the New Economic Zones. Between 750,000 and 1 million were internally relocated.
The zones were often remote sites in the mountains or jungle, far away from urban areas. Families would be forced to relocate, leave their property and belongings, and tend the land. There was often no real infrastructure in these areas, and many died of disease or starvation.
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.
People were required to contribute their crops to feed the military (fighting the Khmer Rouge), and what little was left was rationed to the populace. Farms and plantations were often forcibly collectivized. In addition to the widespread inflation, the forced collectivization and central planning of agriculture were key factors contributing to the famine that wracked Vietnam in the late 1970's and throughout the 1980's. There were an estimated 50,000 deaths in the NEZs.
All government officials, low-level government employees, bureaucrats, politicians, in some cases teachers and university professors, and former and active military personnel were required to report to reeducation camps. In a nutshell, the lower your rank or tenure was, the less time you were to spend in the camps, learning about socialist theory and "atoning" for your role in the war.
The lowest ranks were to spend just a few days, mid-level people were to spend a week or two, and the highest officials were to prepare for up to a month. These men (and women) were told to bring some clothes and personel toiletries, but that was it.
Besides the forced indoctrination, these camps also doubled as hard labor camps, which involved back breaking jungle labor, mine clearing, and other projects. Over 300,000 people were ordered to the camps. Although many were released in a few weeks, many South Vietnamese military personnel, mid- and senior-level officers, would endure the camps for years and even decades. The last South Vietnamese generals were released in the 1990s. Little food or medicine was available in the camps, and as a result, thousands died, buried in unmarked graves in remote jungle locations. An estimated 165,000 died in the camps.
With many unable to find work stemming from their previous government affiliation, and others finding themselves without property or homes, Around 800,000 chose to flee Vietnam, by either the risky overland route into Thailand, or the sea lanes of the South China Sea. Mostly in rickety old fishing boats, often overloaded by two, three, and four times their capacities, the boat people would pay off local government officials or coastal guards in order to leave the country. Families would pay for passage on these boats in gold.
On the ocean, they fended off pirate attacks, government attacks, and violent storms. It is unknown how many were abducted or sexually assaulted and raped by pirates. Lucky boats were rescued at sea by passing freighters or by U.S. naval vessels operating in the area. Many headed for Malaysia or Indonesia, where often times the maritime patrols and coast guards would push the boats back out to sea. In most cases, they were interned in refugee camps for resettlement by western nations. Many died in the camps waiting to be granted asylum. Many more died at sea. UNHCR estimated that 400,000 died, though the range is wide.
Overall, the South Vietnamese people suffered greatly after the war. Disregarding deaths due to famine, warfare with the Chinese and Khmer Rouge, between 1975 and 1987, an estimated 400,000 - 2,500,000 died as a result of political violence. That is R.J. Rummel's figure, and while he has some clear biases, his work on democide is really the only comprehensive study of post-war Vietnamese deaths. I'm inclined to agree with mid-level numbers within that range, as the numbers for extrajudicial killings fluctuates depending on where you read.