How were South Vietnamese, in general, treated by North Vietnam ?
They were treated so terribly that to this very day many pre-75 South Vietnamese (those who had moved to South Vietnam before 1975 to flee the North Vietnamese with some working for the government) hate North Vietnamese with a passion and for good reason: they were treated very harshly.
You have to understand one thing about the Communist Vietnamese (and communist group in general): they had been guerillas who suffered terribly. Most, if not all leaders of the Vietnamese Communist Party (VCP), had been jailed, locked up, tortured. Ho was locked up in Hong Kong, Giap was locked up and his wife was executed by the French, the list went on. It was during those time they learned this lesson:
"Death before dishonor"
When a man believed in something, he would gladly die for that something. You can kill him but you only create a martyr which will inspire many many more to take up his battle cry. But a broken man is more useful: nothing dissuades people more than the sight of a revolutionary broken and humbled.
But how do you break a man ? You can torture him, but do it too hard and you kill him. Do it too gently and he will grow accustom to the pain.
But what if you torture their family ?
Nothing breaks a man faster than seeing his children suffer for his sins. The worse the suffering, the more prolonged the suffering, the faster a man break. You can pull every teeth from a man's mouth and do live appendicitis operation on him and he will spit in your face. But you beat his children and he will give up everything you want.
And the VCP applies this lesson to heart.
After the war was over, the North Vietnamese government initiated "re-education" plan starting June 11th 1975. The re-education was supposed to only last "a few months". For most, it lasted many years. For many, twenty.
It should be noted there was no institutionalize torture because the Vietnamese Communist knows one thing: they have to keep a facade of being the "nice guys" and having people returning from the camp in casket or as a cripple is not a good idea. There was torture, don't get me wrong, but it was more spontaneous and not institutionalized. They used other methods such as letting disease ran amok in the camp or using starvation and hard labor. The aim is not to break the man, however. The aim is to make life so terrible for them they would yearn for their old lives. The more they yearned, the better.
And then came the gut punch
During the whole time the man was there, his family would suffer. His children would be denied access to higher education and would be subjected to intense bullying from his peers and the Party organ in the school (which is to say everyone). If his children were of age, they would be "persuaded" to join the army or the Young pioneers and would be used as cannon fodders in war or used to clear landmines with their hands in New Economic Region. But most importantly, they would be taught to denounce their own parents. The wives and relatives and everyone who is associated with him would be punished by having their ration cards withheld or demoted to the lowest rank and denied access to healthcare or government job (which meant every jobs since private sector was non-existent). This system, called "3 đời 3 họ" (three generations, three family (your family/your father's family/Your mother's family)) is still in use up to this day.
But the sickest one was their allocation of "prison visit". They deliberately kept information about the inmates whereabout hidden from their family, forcing relatives to go on a wild goose chase that took them across the length of Vietnam. These harrowing journeys took months if not years and all the while the wardens let the inmates knew that if they "got re-educated", the wardens would arrange a meeting and help their family reuniting without having to go on such a harrowing trips. Some wardens took things even further, forcing the mothers and wives and daughters of the inmates to have sex with them in exchange for a visit that would never come and then told the inmates as a way to break them. Just imagine being stuck in jail and listen to a warden who told you, to your face gleefully, that he is fucking your daughters every night. I don't know about you but I will break
The list goes on and on. The deaths and physical humiliation of the camp is not the worst part. The psychological is the most effective and the worst. So many men were scarred by this experience that they became avid communists because of Stockholm syndrome while some never spoke of their experience out of fear of reliving such experience again. But that was only one half of the journey.
When these men re-entered society, they found a world unwilling to accept them in. Their own families suffered, their children grew up to hate them. They had little way to make a living in life. And nobody wanted to associate with them since such association would invite the watchful eyes of the dreaded Public Security. Many were forced into suicide. Some became so broken they went mad or died a homeless beggar. The few who managed to keep their minds intact opt to leave the country, but only after they managed to get enough gold and foreign currency to pay the government to let them leave. The sum was mostly so large they had to sell every belongings of them only to venture out as a boat person into the high sea where they would be robbed or killed by pirates.
In all, about 800,000 Vietnamese left Vietnam from 1975 to 1995. Anywhere from 200,000-400,000 of them would die, a mortality rate from 25% to 50%
The policy was flawless. Unlike the Chinese or the Russian, the Vietnamese broke people without leaving any physical trace behind and did so thoroughly those who suffered became unwilling to testify. Therefore they could easily deny that such mistreatment every happen and whoever testified against them would be impugned as "spreading falsehood". They managed to eradicate any threats to them without firing a shot while at the same time hoarded vast amount of wealth.
But most importantly, the program was so great it left a psychological mark on the people. They managed to cow everyone into submission, even the young and hot-headed youth. To this day, people are still afraid to speak out because they know that if they ever ever displease the government, it will not be them who suffers.
It will be everyone they love.
And the government will make sure they get the front seat and watch all those suffering.
People freaked out when they read 1984. Big brothers ain't nothing compared to the VCP
Source
"The winning side" by Huy Đức
"Vietnam's Forgotten Army: Heroism and Betrayal in the ARVN" by Andrew Wiest
Edit: In the spirit of fairness (something that the Northerners never give to us Southerner, nor will they ever bother to learn) there were people who actively working towards reconciliation. Former Prime Minister Võ Văn Kiệt, or "Sáu Dân", was actually sympathetic and tried his best to heal the divide. This was because he was a progressive who had family members fighting and dying on both sides. And since no good deeds went unpunished, he would be subjected to numerous purges. But he managed to survive and he can be considered to be modern Vietnam's second greatest statement after Hồ Chí Minh (and surely the most progressive)
French, South Vietnamese, and American forces were absolutely brutal in how they treated suspected and actual Communists.
Con Dao prison should give a pretty good example of a possibility for a guerilla, POW, or suspect.
A prisoner on Con Dao would be given a sentence of several years and shipped to the island and placed inside a prison similar to a blockhouse prison you see on old colonial possessions in the Caribbean. Very little in the way of human dignity. Concrete or rough hewn stone everywhere, basic iron shackles, really basic and primeval toilets, water sources, and cooking. Prisoners were kept in airless oven-like rooms about 40x40m2 chained to a single metal rod which made sleeping difficult, laying in their own excrement and piss throughout the night. Rooms could contain hundreds of prisoners.
Most prisons contained an element of labor. On Con Dao prisoners waded into the shallow bay to collect coral, diving down to get it, many hardly able to swim, cutting their feet and legs, contracting gangrene and other diseases. The coral was then processed into lime and used to build structures around the prison.
Disease was rampant with most prisoners dying of malaria, typhoid, and infections related to untreated wounds. When the camps were liberated at the end of the war, prisoners looked akin to concentration camp survivors.
The prison housed many of the Communist leadership's highest level members before their executions in the 1940s-50's and remained a very large prison camp until the end of the war. It's where the idea of the barb wire rimmed "Tiger cages" come from as well as the use of "solariums" were prisoners would be left exposed to elements without water for days.
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CID has been implicated any many extrajudicial killings. Guerillas were not considered to be under the protection of the Geneva convention and therefore were seen as someone you could mistreat, torture or otherwise summarily execute. There are reports of American interrogators leaving the room when electric shock was being administered to prisoners by ARVN interrogators only to return so as not to be implicated in torture. These come from second hand comments of CIA officers, other soldiers, and a few vets I've met over the years, so I have not academically sourced them. That being said I wholeheartedly believe they happened and believe someone could pop up with a source easily enough.
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Many POWs switched sides. Sometimes multiple times. "Kit Carson" scouts were often reported to be ex VC. This might mean that they had fought with the Nationalist Vietminh forces earlier in the revolution or it could mean they literally flipped sides to work for the American/ARVN forces. These men were often considered highly useful but also unreliable. They would slip back to their villages and try to melt back into the regular population sometimes, or losing heart after a terrible battle might flip to the winning side. The Viet Cong was often partially made up of former ARVN soldiers who defected to join the other side. So this is to say that POW's might join the other side after a soft touch commander convinced them it was better than the prison camp or forced labor.
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In summary. North Vietnamese POWs were often treated very harshly during the war and could expect little sympathy from the other side. Conditions inside of the POW camps were nightmarish and could be more deadly than some of the combat zones the Viet Cong and NVA operated in throughout the theatre. Intelligence gathering in the South was known to have involved torture and there is some evidence of extrajudicial killings, but also formal executions. Both the North and South allowed prisoners to switch sides and join their forces rather than rot in a POW camp. After the war some of these sites have yielded mass graves in the 10s of 1000s with the dead going back over 50 years. Guards, Wardens, and officers were often executed after the war for the harsh conditions that the North Vietnamese suffered in the camps.
Sources: I have visited the Prison on Con Dao island and read several primary accounts and journals of former prisoners there. The island is course riddled with propaganda by the Vietnamese state, but photographs and the site itself also very telling in making it objectively clear what a horrible place it was. I live in Vietnam and have had the good fortune to meet a few old vets over my years here that have been kind enough to tell me a little bit about their experiences as soldiers here during the war. Some of those stories pertained to the dark and often troubling capture of prisoners. I wish I had better access to a nice research library to give you all a proper works cited, but as of now I can only dig into my brain and tiny bookshelf. Thank you for reading.