What is Vassal?

Legal Definition
A vassal or feudatory is a person regarded as having a mutual obligation to a lord or monarch, in the context of the feudal system in medieval Europe. The obligations often included military support and mutual protection, in exchange for certain privileges, usually including land held as a tenant or fief. The term is applied to similar arrangements in other feudal societies.

In contrast, a fidelity, or fidelitas, was a sworn, unconditional loyalty to a monarch.
-- Wikipedia
Legal Definition
Feudal law. This was the name given to the holder of a fief, bound to perform feudal service; this word was then always correlative to that of lord, entitled to such service.

2. The vassal himself might be lord of some other vassal.

3. In aftertimes, this word was used to signify a species of slave who owed servitude, and was in a state of dependency on a superior lord. 2 Bl. Com. 53; Merl. Repert. h. t.
-- Bouviers Law Dictionary
Legal Definition
In feudal law. A feudal tenant or grantee; a feudatory; the holder of a fief on a feudal tenure, and by the obligation of performing feudal services. The correlative term was "lord."
-- Black's Law Dictionary
Legal Definition
A tenant or landholder under the feudal system. See 2 Bl. Comm. 53.
-- Ballentine's Law Dictionary