The Collaborative International Dictionary of English v.0.48:

Vendetta \Ven*det"ta\, n. [It.]
   1. A blood feud; private revenge for the murder of a kinsman.
      [1913 Webster]

   2. Any feud or contention that is bitter and prolonged;
      however, the deep enmity may be held by only one party to
      the dispute; as, the former Mayor nurtured a lifelong
      vendetta against the candidate who defeated him.
      [PJC]

WordNet (r) 3.0 (2006):

vendetta
    n 1: a feud in which members of the opposing parties murder each
         other [syn: vendetta, blood feud]

Moby Thesaurus II by Grady Ward, 1.0:

64 Moby Thesaurus words for "vendetta":
   acrimony, altercation, animosity, animus, argument, avengement,
   bad blood, bicker, bitter feeling, bitterness, blood feud, brawl,
   broil, conflict, contention, controversy, dispute, donnybrook,
   donnybrook fair, embroilment, enmity, feud, fight, fliting, fracas,
   fuss, getting even, hard feelings, hatred, ill blood, ill feeling,
   ill will, imbroglio, logomachy, open quarrel, polemic, quarrel,
   rancor, reprisal, retaliation, revanche, revanchism, revenge,
   rhubarb, rivalry, row, set-to, sharp words, slanging match, snarl,
   soreness, sourness, spat, squabble, strife, sweet revenge, tiff,
   tussle, vengeance, venom, virulence, vitriol, words, wrangle