Well, after the Vietnam war, it was the strongest military power in Southeast Asia and was poised to conquer the entire region and form a powerful nation------all the other powers did not want that to happen. So China spent a decade to full exhaust the Vietnam.
The conflict is a low intensity, but continuous operation. Due to the sheer difference in manpower, technology and industry capacity, while China only have to use partial military forces, Vietnam is forced to fully mobilize each time. China never ventured far from Chinese border, this means Vietnam doesn't even have a supply advantage. So unlike the US occupation of Iraq, Vietnam actually ended up bleeding more resources than China.
Other major international players, the US and its allies, of course, is happy to see Vietnam being wore down and can't develop its economy or capitalize on its military advantage. USSR is also very conservative on the issue. Judging by USSR's improved relationship with China in the 80s and its non-active involvement with China/Vietnam during the decade, it is most likely that they wish Vietnam to be limited to a medium sized nation as well so they continue to rely on USSR. Other Southeastern nations are happy to save their own hide and rest of the world pretty much have the same opinion.
So the end result is shown in 1990s. By the time China/Vietnam conflict finally ended, Vietnam is a former shadow of itself and any dream of it becoming a major nation has thoroughly evaporated.
Nowadays China, US or Russia doesn't mind Vietnam. It can develop economy as it like, but the bottom line is Vietnam will remain a medium size nation indefinitely.
