Japan will never become a vassal state of China, if that is what you mean, the way it became an occupied nation by the US, after its defeat in WW II and remained dependent on the US for its security needs. You have to admit this is a very unnatural situation in the first place and must change to bring back more sovereignty and independence for Japan. The world has changed since the end of WW II and the cold war era. Change is the only constant in geopolitics.
Having US bases in Japan and South Korea is problematic for many reasons. These bases are not just presence of US troops, missile and air bases, they are also creating dynamics within politics of these two countries that may not be in the long term interest of these two countries. Both Japanese and South Koreans are quite aware of the negative effects of the presence of these bases and are looking for ways to remove these bases after they find alternative ways to ensure their security needs without US presence, as far as I know.
China, as it rises economically and militarily, is also getting increasingly uncomfortable about these bases. We cannot blame the Chinese, the US did not like Russia putting missile bases in Cuba. The situation is not analogous, but the distance is about the same. You get the point. So China also sees that removal of these bases as a national geo-strategic goal.
So here we see that the goals of the three countries population match, all of them eventually want removal of these US bases. But the stumbling block is how Japanese and South Korea can ensure their security against the threat of a rising China.
What I argue is that a rising and more developed ASEAN presents an opportunity for these 3 East Asian powers to creatively use ASEAN to reduce the need for the US presence. Once ASEAN is fully developed and integrated in a partnership with these 3 East Asian countries, then the US presence may become irrelevant.
Question is why should China allow a hostile alliance such as an ASEAN+2 (Japan and SoKo) form around its own Southern and eastern borders. The answer is that it does not have to be a hostile alliance, it can be a friendly alliance (Frenemy if you will), which may become a bulwark against any possible Chinese aggression, but will ally with China on the global scene under the SCO umbrella, just like Russia and former Soviet states do in SCO.
I think it is also in US interest, if local countries can solve their problems locally and it does not have to worry about protecting far away ally's, when they have their hands full with NATO and other obligations in South America, Mid-east and Africa.
I call this a Unite and Empower paradigm following New Regionalism Approach, rather than the old divide and rule paradigm. I describe it in detail here:
http://www.defence.pk/forums/world-affairs/164048-kalu_miahs-new-world-order-road-map-future.html
By unification, I actually meant Korean unification, through a peaceful Sunshine policy, pioneered by Kim Dae Jung:
Sunshine Policy - Wikipedia, the free encyclopedia
I think you have a lot of experience in these countries. I have some experience with Koreans and Korea as well. Koreans detest the Japanese occupation of 35 years. It was worse than the centuries long existence they had as a Vassal state of the Chinese empires since Silla joined hand with Tang to defeat Beakje and Goguryeo. So there is no question of the Koreans unifying with the Japanese, but I believe they will not mind to partner with Japan in a ASEAN+2 and work with Chinese to integrate ASEAN and make it into a developed region. As it is there is already a lot of Korean and Japanese investments in these countries (ASEAN). Chinese investments have also started to flow in recent years.
The big question is whether Chinese polit buro will see the merit in such an approach, or they will consider it too risky. As for your comment that "Chinese polituro have to be crazy to want to see a country closer to him get stronger......", both Japan and South Korea are already developed countries and the clock cannot be turned backwards. Its only a matter of time that they will grow their own defense muscle, once they move away from US dependence. So Chinese politburo may not have a choice on this matter, rather they would have to seek ways to find the most advantageous position for China in this affair, with cooperation rather than confrontation, so eventually Japan and a unified Korea and ASEAN+2 moves towards SCO and away from US dependence.