Forgive my ignorance on this topic, but I know basically nothing. I saw a post in r/all about the Vietcong camouflage techniques, and ended up seeing a post here about Vietcong tunnel systems used to trap & attack US forces. I don’t know how accurate it is, & it sounds hellish to be living as a Vietcong guerrilla soldier based on this graphic. I’d love some more context about the prominence of tunnels, and how much time people actually spent in them. Is it just my comfortable lifestyle that makes this sound so terrible, or was it less terrible living from the Vietcong perspective compared to mine?
The broad answer is that it depended greatly on where exactly you are talking about. Tunnels were used extensively during the Vietnam War (dating back to the French conflict, where they proved their usefulness). The images you show are relatively accurate, but only for a select area of South Vietnam in the area usually referred to as Cu Chi. This area was notorious for its tunnels, particularly the area known as the Iron Triangle. Reason being, it was close to Saigon, and the nature of the clay in the region made it near perfect for the construction of massive tunnel complexes. Normal dirt does not hold up well digging so much with extra structural support, and is very prone to flooding. Most places, if you dig 30' down you'll hit water, so you have to be able to get around that. The clay in the Cu Chi area did not flood, and had a lower water table. Most tunnels are also prone to cave ins, especially with use of high explosives, but the Cu Chi tunnels were not. The pressure from explosives in some ways actually strengthened the walls by compacting them.
The main reason for the tunnels was American air superiority, and artillery. Because we could call in air support, via helicopters or other aircraft, fire missions literally within seconds, the NLF (National Liberation Front, usually called the Viet Cong by Americans), had to go somewhere to be hidden. In I Corps, near North Vietnam, Viet Cong forces would blend into the local community if possible or into the jungles. Tunnels existed everywhere, but in most cases outside of the Cu Chi area, they were less massive, but still there.
Americans never fully understood just how bad their tactics were in dealing with tunnels. The assumption that you can use gas, explosives, and very brave men with revolvers and clear a tunnel only appeared to work. Water traps, similar to the U joint under your sink, were used to trap gas. Explosives only blocked superficial aspects of the tunnels. Even the ubiquitous Tunnel Rats, brave as they were, still were barely scratching the surface (no pun intended) of the complexity of these tunnels. Booby traps abounded in them. The simple ones could be waiting for an American to crawl up through a hatch and stab him, or use of grenades, or even venomous snakes tied to a bend in the tunnel. Even as Americans began finding amazing things, like full size hospital wards, artillery pieces, and even a tank, once, they never grasped how little of an impact they were making. The 1st and 25th Infantry divisions spent the war trying desperately to destroy tunnel complexes and never really succeeded in any of it. The only thing that came close to being destructive on a large scale were B52s, and even then, they were hit or miss on that. Americans tried defoliants, so we could see the entrances, and that proved superficial as well. We also came up with seismic devices designed to resemble a plant that worked (sort of). Point is, whatever we tried, at best it delayed them, and likely cost many lives in the process.
But to another part of your question, the tactics used by Americans meant the fighters living in the tunnels were forced to stay underground for longer periods of time. And living in the tunnels, compared to our standard of living, and typically any human standard, was difficult. Bugs, lack of sunlight, inadequate fresh air, and especially bacteria meant hospital conditions and clean water were scarce in many areas. How do you get new food if you can't get to the surface? You ration the small bits of rice you have. They created a type of kitchen to be able to cook underground with fires, but these still were inadequate to the real needs. Some literally spent years in the tunnels with only a handful of chance to ever see anything above ground. Children were even born in the tunnels. From the Vietnamese perspective, this was hellish, but was a price to paid. They also tried to keep life as lively as they could, even having entertainers come in and had shows put on for fighters undergound.
Again, most NLF fighters weren't living in tunnels all the time, but its likely most spent time in a tunnel somewhere for some length of time. But especially around Cu Chi, the tunnels were really the battlefield.
For sourcing, I'd of course recommend The Tunnels of Cu Chi by Tom Mangold and John Penycate. Its still the seminal work on tunnels. But just about any Vietnam account by anyone I've every read mentions use of tunnels, even if its just a bunker under a pig pen to store weapons or rice. The Viet Cong and NVA used the earth the way Americans used the skies.