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Via Expectations of the law in the middle ages
A liege vassal was somebody who was completely his lord's man
a vassal who will serve his lord before all others
By comparison, a simple vassal is a vassal in the traditional sense: of the king first and foremost, then perhaps to one or more dukes or counts under the king (or as the case may be, to multiple kings!). His loyalty to the duke above him only extends as far as his land within that duke's realm.
Both are distinct from a subject, who in Bodin's definition is inherently beholden to a lord, whereas vassals of both kinds only owe this fealty while they hold the lord's fiefs that they have been entrusted with.