How could Sino-Vietnamese relationship being a good example after Vietnam's reunification under North Vietnam? Soon after North Vietnam took over South, it went on to conquer Cambodia (another Chinese ally in the region) and forced Laos into its orbit (another country having a good relationship with China in the region), and in the mean time it (Vietnam) allied with USSR (the then No.1 enemy of China). If anything, the Vietnamese example proves the opposite.
Regarding the possible reunification of Korea, you are suggesting under SK, right? A reminder, last time when China helped a southern Korean power was in the early Tang dynasty, the Korean power was 新罗/Silla. Tang paid the cost and lives to conquer 高句丽 and 百济, only for the lands to be gradually sliced away by Silla. In the end Tang paid the bill for Silla to gain almost everything (south of Pyongyang of 高句丽). This time around, I think China should get smarter, not footing the bill. If SK has the ambition to be the new Silla, do it themselves.