The Collaborative International Dictionary of English v.0.48:

Vassal \Vas"sal\, n. [F., fr. LL. vassallus, vassus; of Celtic
   origin; cf. W. & Corn. gwas a youth, page, servant, Arm. gwaz
   a man, a male. Cf. Valet, Varlet, Vavasor.]
   [1913 Webster]
   1. (Feud. Law) The grantee of a fief, feud, or fee; one who
      holds land of a superior, and who vows fidelity and homage
      to him; a feudatory; a feudal tenant. --Burrill.
      [1913 Webster]

   2. A subject; a dependent; a servant; a bondman; a slave.
      "The vassals of his anger." --Milton.
      [1913 Webster]

   Rear vassal, the vassal of a vassal; an arriere vassal.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Vassal \Vas"sal\, a.
   Resembling a vassal; slavish; servile.
   [1913 Webster]

         The sun and every vassal star.           --Keble.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Vassal \Vas"sal\, v. t.
   To treat as a vassal; to subject to control; to enslave.
   [Obs.] --Beau. & Fl.
   [1913 Webster]

WordNet (r) 3.0 (2006):

vassal
    n 1: a person holding a fief; a person who owes allegiance and
         service to a feudal lord [syn: vassal, liege,
         liegeman, liege subject, feudatory]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "vassal":
   bondmaid, bondman, bondslave, bondsman, bondswoman, captive,
   chattel, chattel slave, churl, client, concubine, creature,
   debt slave, dependent, feudal, feudatory, flunky, follower,
   galley slave, hanger-on, helot, homager, inferior, lackey, liege,
   liege man, liege subject, minion, myrmidon, odalisque, peon,
   retainer, serf, servant, servile, slave, stooge, subject,
   subordinate, subservient, theow, thrall, tributary, underling,
   understrapper, villein, yeoman



Bouvier's Law Dictionary, Revised 6th Ed (1856):

VASSAL, 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.