Japan was never a vassal of China.
The empress Suiko of Japan started sending missions to China by 600BC to learn technique for governance ,China's emperor Yangdi,after unifying substantial part of China for the first time in centuries, wanted formal relationship with nearby states after coming to power.
Empress Suiko had passed a memorial to the emperor of China which mentions "The child of heaven where the sun rises addresses the child of heaven where the sun sets"-that's not how vassals talk ,empressess Suiko seeked respect for japan and wanted the emperors from both nation to be treated as equal.
Japan used to send tributes to japan from 600 till 838 bc, it was a mean for trade concessions by China,after the decline of Tang ,China's appeal to the Japanese had declined and Japan stopped sending tributes by 838 bc.Japanese had adapted what they learned from China according to Japanese needs & taste by then .
Though Japan had again send tribute to China in 1403 for trade benefit ,but stopped that in a century.
China repeating the mistake of disrespecting Japan.