Millions of casualties, trillions of won lost and decades of suffering under the "Reconstruction" era.
My answer to the post-war situation is military occupation and government like with Germany and Japan, keep the borders sealed, beg borrow or steal as much food and humanitarian aid as you can and run it into the country to try and help stabilise things, use the incredibly cheap labour you now have on hand to undercut China and other countries in the mass production market like with the current Kaesong Industrial Region, use that income to help fund reconstruction and education of the population over a couple of generations, over the 40-year period start a gradual introduction of democracy and finally unification.
China is going to be as nervous as hell about a unified Korea under the South. I think they could probably be reassured if it was promised to be fully nuclear disarmed under South Korea's commitment as a signatory of the Treaty on the Non-Proliferation of Nuclear Weapons, with the war now effectively concluded it means that the UN Security Council resolutions have been fulfilled so that United Nations Command can be disbanded and US troops asked to leave the peninsula. There's also the factor that North Korea is a headache even for the Chinese government that I think seeing the UN, really American, forces gone would be seen as a decent trade-off.
Except there would be the added problem of insurgency by forces loyal to the ex regime including NK Special Forcs types. This woul be based in the extensive mountainous and forested areas and likely would last for years. Much like Iraq
I think the Vietnamese soldiers who posted on Facebook may not be aware of the state of the North Korean Weapons. Vietnam has had the ability to continue to keep its equipment running and likely still gets some stuff from Russia and other nations. The North Koreans haven't had any external support in years and their gear is largely third hand stuff, hand-me-down hand-me-downs. Their most advanced jets are a squadron of early model MiG-29s, and reports indicate they rarely use them except for scant time to qualify their pilots.
In essence once the MIGs are out the KPA is really an early to mid Cold War Eastern Bloc army.
How long can they count on popular support? My guess is until ROK troops start distributing food.
Consider how indoctrinated the NK population is. It would take years to change that. Just look at Iraq. In maybe ten years or so you might get popular opinion starting to shift in your favour. But would the US be willing to sustain the financial costs and the caualties of a long term COIN? The conventional war, nasty and boody as it would be, is very likely the "easy" part![]()