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Just to clarify: is your question about why so few vassals ultimately took over their respective overlords’ kingdoms? Like, “Why did the throne of France never fall to a component duchy like Aquitaine or Brittany?”

Firstly, I would argue strongly against the idea of "tendency towards centralization." Not every country centralized, nor did they centralize the same way. For instance, the English Parliament arguably gave the vassals more power than they did in other countries, and in the end today I doubt anyone would imagine the monarch of Great Britain having significant political power. Similarly, the Polish-Lithuanian Commonwealth is known for having weak rulers and extremely powerful vassals, as was Sweden and the Age of Freedom, and so on and so forth. Even as late as the 1800s, the Austrian empire devolved significant powers to the Hungarian Diet, which again is a trend away from centralization.
Nor did being a vassal stop people from annexing other countries. The most famous example of course being William the Conqueror, who seized the throne of England despite being a vassal of the King of France (which in turn set off all sorts of fun interactions with inheritance law), but there were many instances of "vassals" taking over large territories, like the Electorate of Brandenburg also ruling over Prussia, the Duchy of Burgundy gradually expanding his titles into the Holy Roman Empire, and so on and so forth.
The reason why conflicts of vassals against their liege were not the total war that more or less took place in China was for multiple reasons, but some leading contributors included the nature of the relationships between liege and vassal as well as the incentive for lieges and vassals to work together.
In Europe, there was an implicit relationship between a king and his vassals in that the vassals could petition the king for help with other affairs. For instance, in the famous duel between Jean de Carrouges and Jacques de Gris, Jean, a vassal of the Count of Normandy, petitioned the king for the right to hold a judicial duel against Jacques de Gris, as Jacques was a close advisor of Jean's de jure lord, the Count of Normandy, who was protecting him from Jean. In effect, there was a whole system of rules and obligations between vassals and their lords that made it a mutually beneficial relationship. Some vassals would also use personal diplomacy to increase their standing at court, and similarly vassals had certain protections against power from the king. In addition, wars were usually more limited in scale as neither side could afford to maintain the wars for long.
In China there was no such system. Prefectures were less "vassals" and more "tributaries" in the sense that the central government had next to no obligations to help or defend any particular part of China. Essentially, the only real set of obligations the central government had to its regional areas was to send tax collectors to take a share of the tax the local region had collected, as well as to occasionally provide support for particular initiatives the central government wanted like flood control or fort construction. Defense was relegated to the border provinces and there was very rarely a powerful central military. Moreover, a "vassal" that did not want to do what the central government wanted faced either being sacked or execution, probably both. They had no protections and no benefits from the central government, so often times the only way out for them was to revolt. And when they did revolt, since there was no reason for them to surrender (as they would likely just be executed anyways), the war was much more "total" in scale. There were exceptions to this of course (ex. the Three Kingdoms period, where the powerful warlords had a feudalesque relationship with their retainers). That said, if one was going to rebel, the only way the war could end would be by taking over the country. So it would be inevitable that only by subjugating both the central government AND the other regional vassals that one could take over.

>Why aren't the vassals in Europe, especially Western Europe, keen to annex entire countries?

I would dispute that this is the case.

Firstly, European rulers throughout the Medieval period had this concept of being heir to the Roman empire ... from the rulers in Constantinople, the Karolingian rulers, the Habsburgs, even Russia got the idea ... each of them, if in the right circumstance, would have jumped at the opportunity to claim this heritage and rule over Europe.

Secondly, West-European rulers did have ambitions of conquest. The aforementioned Karolings being arguably the most successful ... but then also you have the 100 Years War, The Reconquista, the 30 years war even ... European rulers took every opportunity to try to take more land to add to their domain.

Napoleon, although not a medieval ruler, is also an example for both points from above.

The thing is, conquering isn't easy. They all relied on right circumstances to enable their conquest.

In my opinion, the better question would be how come domains in Europe tended to break apart and divide while Chinese tended not as much?

And this is where the difference in systems of governance and laws come in play. I believe the key differences was made by the presence (or lack of) of the bureaucratic apparatus as well as title inheritance laws. All systems are self-serving and self-perpetuating, they favor action which will result in their continuous existence.

The Chinese bureaucrats' survival depended on the keeping of the Imperial institution alive, but you could have only one of those ideally ... otherwise the Mandate of Heaven goes out the window. So governance was directed towards bringing everyone under the same authority.

Europe didn't have that. Europe had the top ruler binding vassals (and be bound) by feudal laws/contracts and what would be bureaucratic officials in China, here those or similar functions were occupied by vassals. But the vassals' survival and quality of life depended on how beneficial those feudal relations were, and those could change. If the relationship didn't do any good for the vassal, the vassal could decide simply to go their own way, attack neighboring vassals or even try to topple the ruler. By doing so successfully their position in this world would improve.

The bureaucrats in China would not gain much by utilizing the same approach.

...

>and enlightened autocracy in the later period

btw this is thanks to Voltaire learning about Confucius from the Jesuits and becoming a fan. He even asked Catherine the Great to bring in scholars from China to Russia to help her more (didn't happen afaik).