The main issue I see with this is that Wang had a very long-standing tendency, dating back to the early 1930s, towards the viewpoint that China needed to accommodate itself almost completely to Japan in order to avoid conflict and build itself up economically and militarily until it could at some point in the distant future defeat the Japanese or at least overwhelm them economically, which was probably related to his aversion towards allying with the other major powers in the fear that they would themselves subjugate China if the country tried to use them against Japan. It wasn't just a spontaneous decision in 1938 that resulted in him running off to Japan, but rather the culmination of these beliefs, and that makes it a bit more difficult to avert than something like a plane crash or car accident.
So what you really need a PoD that can avert this feeling and make him have attitudes more similar to Chiang, who instead thought that China would be able to win against Japan soon and that the Soviet Union, France, Britain, and the United States could be useful partners against Japan, hence it would make sense for China to hold out. I can see several possibilities for this. The first, perhaps most obvious, would be Chiang's army proving much more successful against the Japanese and quickly stalling them before they could capture much of the country instead of mostly failing or being destroyed. This would obviously validate Chiang's views and probably convince Wang to stick with the RoC instead of defecting, even if it becomes apparent that Chiang's army can't really win the war either. Of course, this would probably require a 1920s or earlier PoD to allow the RoC to have a much more effective army, Hearts of Iron notwithstanding.
The second, a bit less obvious, would be to change Wang himself. If you can arrange for him to be more of a partner with Chiang than a rival, or for him to come out on top in the struggle for power after Sun Yat-Sen's death, then he might tend more towards Chiang's viewpoints, if for no other reason than sheer practicality. But this is a bit more elusive than the other; it might make the "defection" irrelevant because he starts running the RoC like a Japanese client state to avoid a war, for example, which would technically be fulfilling the OP but not the way the post wants (though perhaps in an interesting way; if the Japanese win without fighting, what do they do next?)