The Collaborative International Dictionary of English v.0.48:

Vindicate \Vin"di*cate\, v. t. [imp. & p. p. Vindicated; p.
   pr. & vb. n. Vindicating.] [L. vindicatus, p. p. of
   vindicare to lay claim to, defend, avenge. See Vengeance.]
   1. To lay claim to; to assert a right to; to claim. [R.]
      [1913 Webster]

            Is thine alone the seed that strews the plain?
            The birds of heaven shall vindicate their grain.
                                                  --Pope.
      [1913 Webster]

   2. To maintain or defend with success; to prove to be valid;
      to assert convincingly; to sustain against assault; as, to
      vindicate a right, claim, or title.
      [1913 Webster]

   3. To support or maintain as true or correct, against denial,
      censure, or objections; to defend; to justify.
      [1913 Webster]

            When the respondent denies any proposition, the
            opponent must directly vindicate . . . that
            proposition.                          --I. Watts.
      [1913 Webster]

            Laugh where we must, be candid where we can,
            But vindicate the ways of God to man. --Pope.
      [1913 Webster]

   4. To maintain, as a law or a cause, by overthrowing enemies.
      --Milton.
      [1913 Webster]

   5. To liberate; to set free; to deliver. [Obs.]
      [1913 Webster]

            I am confident he deserves much more
            That vindicates his country from a tyrant
            Than he that saves a citizen.         --Massinger.
      [1913 Webster]

   6. To avenge; to punish; as, a war to vindicate or punish
      infidelity. [Obs.] --Bacon.
      [1913 Webster]

            God is more powerful to exact subjection and to
            vindicate rebellion.                  --Bp. Pearson.
      [1913 Webster]

   Syn: To assert; maintain; claim. See Assert.
        [1913 Webster]

WordNet (r) 3.0 (2006):

vindicate
    v 1: show to be right by providing justification or proof;
         "vindicate a claim" [syn: justify, vindicate]
    2: maintain, uphold, or defend; "vindicate the rights of the
       citizens"
    3: clear of accusation, blame, suspicion, or doubt with
       supporting proof; "You must vindicate yourself and fight this
       libel"

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "vindicate":
   absolve, account for, acquit, advocate, amnesty, argue, assert,
   bear out, claim, clear, confute, contend, cry sour grapes,
   decontaminate, defend, destigmatize, discharge, dismiss,
   dispense from, disprove, do justice to, exculpate, excuse, exempt,
   exempt from, exonerate, explain, forgive, free, give absolution,
   grant amnesty to, grant immunity, grant remission, guard, justify,
   let go, let off, nonpros, pardon, plead for, protect, prove, purge,
   quash the charge, rationalize, redress, refute, rehabilitate,
   reinstate, release, remit, restore, revenge, second, set free,
   shield, shrive, spiteful, support, uphold, warrant, whitewash,
   withdraw the charge