Why would Japan want to switch to being a Chinese vassal. Having a dominant, powerful china right next door is good for Japan's strategic environment? Japan is more concerned about China than America.
Because China wouldn't depend upon Japan's misfortune to thrive in general as the US depends upon, alongside other vassals too, and even increasingly so, as China is the one who is truly exceptional and doesn't need to rely on outside plunder to develop (Western colonial model) when it has 1.4 billion smartest people on the planet and can develop on its own, as it was the case for the majority of the recorded history when it was the most successful power in continuity, by measuring years on top in total, not just last 500 years.
Why would they exploit Japan, as the US, then? They have better things to do - develop and invest internally to the fullest. They have the best human, social, and political capital to thrive on their own, in the world, whereas the US has the opposite, it just has a lucky-gained hegemonic position (alongside the entire West) it wants to hold one for which it cannibalizes even its allies/vassals fully now.
That's why they are so "insecure" also, so they go starting hundreds of wars all over the planet to preserve that hegemony, hundreds of trade restrictions, starting tech wars, threatening countries to use dollar and treasuries, and everything else in between, whereas China isn't insecure, it has the capabilities to be stronger than anyone internally, so why would it be threatened by Japan and continuously pummel it down as the US?
So, they don't need to put any 'voluntary export quotas', forced manipulation of currency, or forced monetary policy so Japan now needs to go to Washington first to ask if they can sell their UST holdings or de-dollarize part of their trade in energy, forced economic model transition for their capitalists to exploit, forced military occupation with bases above law, forcefully using Japan as the "launchpad" against China, to be destroyed, etc. The US/West, in general, depends on exploiting others to thrive, even its allies/vassals, whereas China develops on its own.
That's what's an objective reality, everything else is CIA controlling Japanese politicians, mass Stockholm Syndrome, nuclear bombs, media propaganda brainwashing, etc. But even with all of this, there are probably still many Japanese who could see what I'm talking about.
And btw, China was "dominant" throughout its entire history over Japan, and they didn't even invade them once, Japan invaded them in the end instead.
So your "logic" then is that it is a better situation for Japan to be punched in the face continuously every day by the US, as opposed to trading that for actually no punching anymore and a very little chance (looking historically) that China might "attack" them one day for no reason?