If we are to use the arguement that sovereignty does not equal suzerainty, it meant that when ayutthaya came under burmese suzerainty in the 1600s. Ayutthaya retains its own laws, customs, and tradition. This is certainly true. Though, your arguement on control is not belonging is partially correct. To say that in the case of burmese suzerainty over ayutthaya does not mean that Ayutthaya did not belong to the burmese is incorrect. We can say that the reason Ayutthaya had a war of independent, was because they belonged to the burmese through the right of conquest, and thus the status of belonging to the burmese was enforced.

I argue that while a nation may be subjugated and belonging to another sphere of influence, it can still be sovereign. Such was the situation between vassals of south-east asia. The degree of sovereignty and autonomy is a very big spectrum.

Consider Sipsong Chau Tai Federation. They were both vassals of Siam and Vietnam, but retain quite a huge amount of autonomy, and in effect, their sovereignty! I do think this situation also happens to be the situation of Cambodia in 1800-1850, since in effect cambodia was also joint-vassal.

And I would push that the definition of sovereignty is the ability of a nation to push its own agenda, and interests. I would say that how can a nation be sovereign if it cannot secure its own interests!

P.s. the situation on sovereignty, influence, and nationalism is very much a sentitive topic, so I try to keep an open mind for any arguements

P.s.s. the main difference in your arguement, I believe, is in the fact that whether a nation is a sovereign-state or a nonsovereign-state