Originally posted by Cockney
Yeah great another country for China to buy 
I've been thinking about this problem quite a bit actuaslly asnd I think I have found a solution.
Dai Viet, Japan, Ayuthaya, and Korea should be almost impossible for China, or anybody else for that matter to hold if conquered or annexed. and the solution would be to create a permanent equivalence to the dutch revolt for these states.
Before anyone says anything about how this would be silly. I think that the histories of these non chinese e. asian states speaks for itself.
All were fully aware of the existence of an expansionist Chinese state for in some cases up to 2000 years at this point. All these states were masters of resisting this expansion and fouind very effective weays of dealing with it. Two of these states Korea and Dai Viet were at one point partially integrated in to the Chinese state, yet both of these states managed to recover independence and create strong centralized states which convinced the Chinese that they were two tough to interfere with.
During the Han dynasty Korea and Northern Vietnam were conquered and made part of the Chinese state, but after the fall of Han, both managed in rather impressive revolts to free themselves. The various Korean states so successfully that no other signifigant chinese attempt was made to conquer them and Chinese armies (the Mongols don't count here) only intervened in Korea at the behest of the Koreans. When the Japanese invaded in this time period (late 16th cent.) they were resisted in a massive war that thopugh won with Chinese assistance showed the entrenched "nationalism" of the Korean people nobles and commoners alike.
Vietnam is a slightly different case, and there the instititutionalization of "revolt" is on even surer footing. At the end of the Han, Vietnam established a pattern of revolting every time the chinese state weakened and was re pacified by armies sent south to restore order. This cycle of revolt and pacification was so pronounced that the traditional chinese name for Vietnam was An-nam (or pacified south) an optimistic notion if their ever was one. After about a millenia of this the Vietnamese revolted in the chaos at the end of the Tang dynasty (late 9th century) and this time the hard pressed Song dynasty was reluctant to be involved in such a losing propsition and accepted Vietnamese independence. In the 15th century their was a civil war in Vietnam and the then powerful and resurgent Ming dynasty made a serious attempt to intervene. Their goal seems to have been something less than the proposed military vassalization. But they quickly were driven out by the Vietnamese.
The point of all this is that the two surviving states on the Chinese periphery survived becasuse they were to tough to conquer and though in the case of Korea a symbiotic relationship similar to vassalization existed it was at the suifferance of the Koreans. Their were plenty of non chinese states bordering china that did not survive, Dali and the Xia in the medieval periodfor example, but those that did survive survived because they were not conquerable.
