In feudal society, a vassal was a man who gave military service to a lord, in return for which he was protected by the lord and received land to live on.
N-COUNT
附屬國
If you say that one country is a vassal of another, you mean that it is controlled by it.
例句
Opponents of the treaty argue that monetary union will turn France into a vassal of Germany.協定的反對者認為貨幣聯盟會使法國淪為德國的附庸國。