Real Dictionary

vassal
If you say that one country is a vassal of another, you mean that it is controlled by it.
countable noun: 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.

vassal in American English
in the Middle Ages, a person who held land under the feudal system, doing homage and pledging fealty to an overlord, and performing military or other duties in return for his protection; feudal tenant
of or like a vassal; dependent, servile, etc.
adjective: of, pertaining to, or characteristic of a vassal
noun: a person granted the use of land, in return for rendering homage, fealty, and usually military service or its equivalent to a lord or other superior; feudal tenant

vassal in British English
adjective: of or relating to a vassal
noun: (in feudal society) a man who entered into a personal relationship with a lord to whom he paid homage and fealty in return for protection and often a fief. A great vassal was in vassalage to a king and a rear vassal to a great vassal