LOL............. do you even know what being a vassal state means? It means for the past 1000 years China simply tells the Korean king to send over silver, Korean girls and an emissary to do the prostration in front of the emperor whenever China needs something. That's called enslaving Koreans.
Do you remember what happened after your Battle of Salsu? China got together with Silla and destroyed Goguryeo, including sacking Pyeonyang in 669. I assure you the Chinese army got their revenge against the Goguryeo people.
Do you know why Koreans are always being killed by Japanese, Americans, Manchurians, Mongols and other Koreans? It's because China offers Korea as a meat shield to weaken potential enemies of China. Sometimes, we use Silla to kill Goguryeo, sometimes we use North Korea to kill South Korea, sometimes we offer Choseon as a sacrifice to slow down Imperial Japan.