After the war from 1609, Ryukyu remained officially a vassal of China, not of Japan, even if it became secretly also a vassal of the Satsuma domain from Japan (not of the Japanese state).
This dual allegiance of Ryukyu, openly to China and secretly to Satsuma allowed Ryukyu to be an intermediary in some commerce between China and Japan, which officially was forbidden.
The official occupation of Ryukyu by Japan happened only in 1872, after the Meiji Restoration.
After 1609, there was no occupation of Ryukyu by Japanese. There was only a permanent threat of military intervention from Satsuma if the Ryukyuan king would have dared to act against the demands of Satsuma, which included a tribute and unfavorable commercial relationships.
Not sure where you're getting "secretly" from since there was no secret and they were openly obedient to Japan. There's nothing outside of CCP sources, and particularly no historical sources, that could even remotely claim otherwise.
And even if you goalpost shift to the Meiji Restoration, 1872 is still well before any period where Japan was required to give up any territory as a result of WW2 imperialism.
This dual allegiance of Ryukyu, openly to China and secretly to Satsuma allowed Ryukyu to be an intermediary in some commerce between China and Japan, which officially was forbidden.
The official occupation of Ryukyu by Japan happened only in 1872, after the Meiji Restoration.
After 1609, there was no occupation of Ryukyu by Japanese. There was only a permanent threat of military intervention from Satsuma if the Ryukyuan king would have dared to act against the demands of Satsuma, which included a tribute and unfavorable commercial relationships.