The Collaborative International Dictionary of English v.0.48:

Vanquish \Van"quish\ (v[a^][ng]"kw[i^]sh), v. t. [imp. & p. p.
   Vanquished (v[a^][ng]"kw[i^]sht); p. pr. & vb. n.
   Vanquishing.] [OE. venquishen, venquissen, venkisen, F.
   vaincre, pret. vainquis, OF. veintre, pret. venqui, venquis
   (cf. an OF. infin. vainquir), fr. L. vincere; akin to AS.
   w[imac]g war, battle, w[imac]gend a warrior, w[imac]gan to
   contend, fight, OHG. w[imac]gant a warrior, w[imac]gan to
   fight, Icel. v[imac]g battle, Goth. weihan to fight, contend.
   Cf. Convince, Evict, Invincible, Victor.]
   1. To conquer, overcome, or subdue in battle, as an enemy.
      --Hakluyt.
      [1913 Webster]

            They . . . vanquished the rebels in all encounters.
                                                  --Clarendon.
      [1913 Webster]

   2. Hence, to defeat in any contest; to get the better of; to
      put down; to refute.
      [1913 Webster]

            This bold assertion has been fully vanquished in a
            late reply to the Bishop of Meaux's treatise.
                                                  --Atterbury.
      [1913 Webster]

            For e'en though vanquished, he could argue still.
                                                  --Goldsmith.
      [1913 Webster]

   Syn: To conquer; surmount; overcome; confute; silence. See
        Conquer.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Vanquish \Van"quish\, n. (Far.)
   A disease in sheep, in which they pine away. [Written also
   vinquish.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

vanquish
    v 1: come out better in a competition, race, or conflict;
         "Agassi beat Becker in the tennis championship"; "We beat
         the competition"; "Harvard defeated Yale in the last
         football game" [syn: beat, beat out, crush, shell,
         trounce, vanquish]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "vanquish":
   bear down, beat down, bend, break, conquer, crush, defeat, fell,
   flatten, humble, master, overpower, override, overturn, prostrate,
   put down, quell, reduce, ride down, smash, subdue, subjugate,
   subvert, suppress, surmount, trample, trample down,
   trample underfoot, tread underfoot