Korean war and Vietnam war were both civil wars with interference from 3rd parties. Those wars were not invasions. That must make a huge difference in required man power, tactics and secure areas.
Frankly the only country that utilized a civil war to succeed in its objectives is India. Nice clean separation of Bangladesh from the Porkland. And permanent peace in north east. Plus the separation of Bangladesh has sounded the death bell for Porkland’s economy since then as they never could govern well with oppression and carnage done by decades of military rule. With one strike, india secured peace in its northeast, gained an ally in Bangladesh, and permanently weaken Pakshitstan on the west. This effect is apparent today as Pakshitstan is getting closer to balkanization itself. As India rises, the chances of survival of Pakistan as one country diminishes. This is a real threat for the CCP as they lose a means to control India. It is already happening. Nepal is the same way. They cannot lift a finger against us as the terai population there is completely pro india, and india can use that love and support to hold a referendum there to merge with India, again depriving CCP of all leverage.
At the end of the day all countries in the Indian subcontinent have sizable populations like the Baluch people , Shias and Buddhists of Gilgit Baltistan, Tamils of Sri Lanka, Terai Hindus of Nepal, etc that would gladly break their countries and merge with India. Until India hold these close civilizational cards, the PLAdyboys can do very little even if they bribe the corrupt leadership in all these countries.
In fact as India grows, the Tibetan movement is going to become a giant pain for the CCP, as Tibetans are dharmic people and will never integrate the Hans. That is a serious threat to the CCP.