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.
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   Rear vassal, the vassal of a vassal; an arriere vassal.
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The Collaborative International Dictionary of English v.0.48:

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

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]