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.
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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]

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