South Korea did turn out pretty successful but I don't think the regime was ever as unpopular as the KMT.
South Korea had the advantage of having been a war zone at the end of WWII and during the Korean War plus the Kims in North Korea were seen as just as legitimate as the Southern regime. South Vietnam collapsed as much due to the popularity of Uncle Ho as the unpopularity of the Southern regime (indeed, things only really began falling apart when Diem was overthrown; making South Vietnam merely a glorified playground for the US Military).
Also, there was no mechanism put in place to reunite North and South Korea like in Vietnam that was patently ignored by the South due to the popularity of Uncle Ho.
As you might have guessed, I hate it when people compare Korea to Vietnam.
As for KMT China, they would have the support of the wealthy West and the KMT would milk it for all its worth in order to develop China. I'd imagine that KMT China would would maintain reasonably good relations with the West up to the mid 1970s with a gradual cooling of relations (though barely noticeable) through the late 70s into the 1980s. The turning point would be the fall of the Soviet Union, with the only other major power (Russia) being brought down, China could now begin to fill in that role.
My argument for this is while KMT China wouldn't have to deal with Mao's policies, it would have to deal with corruption and centralization issues, which would have similar, if less flashy, effects on China. I do believe that reform is possible, but it will take time. However, it would have access to Western markets and be recognized by the West as China from the get go.
By 2017, I'd imagine KMT China would be similar to the PRC from OTL.