The term is outdated and rarely used in daily conversation, but you might encounter it in historical contexts. He studied the feudal system where lords had vassals.
Pro Tip 2/3
Hierarchy Implication
'Vassal' implies a clear hierarchy; the vassal is always lower in status than the lord. The vassal swore fealty to his sovereign lord.
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Feudal System
'Vassal' is closely tied to the feudal system, so it's often used in discussions about medieval history. Medieval vassals were granted land in exchange for service.