The Collaborative International Dictionary of English v.0.48:

Vindicator \Vin"di*ca`tor\, n. [LL., an avenger.]
   One who vindicates; one who justifies or maintains. --Locke.
   [1913 Webster]

WordNet (r) 3.0 (2006):

vindicator
    n 1: a person who argues to defend or justify some policy or
         institution; "an apologist for capital punishment" [syn:
         apologist, vindicator, justifier]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "vindicator":
   Nemesis, advocate, apologete, apologist, apologizer, avenger,
   champion, defender, guard, justifier, paladin, pleader, proponent,
   protector, revanchist, successful advocate, supporter, the Erinyes,
   the Eumenides, the Furies, upholder, whitewasher