The Collaborative International Dictionary of English v.0.48:

Vested interest \Vest"ed interest\
   1. a special personal interest, usually financial, in an
      existing system, law, or institution, which hinders a
      person from making objective decisions regarding that
      system, law, or institution. A vested interest may be one
      which benefits a relative, or, in an extended sense, one
      which defends a person's own reputation or previously
      expressed views.
      [PJC]

   2. a right given to an employee by a pension plan, which
      cannot be taken away.
      [PJC]

   3. pl. the persons, corporations, or other groups which
      benefit most (usually financially) from the existing
      system of institutions, laws, and customs.
      [PJC]

WordNet (r) 3.0 (2006):

vested interest
    n 1: (law) an interest in which there is a fixed right to
         present or future enjoyment and that can be conveyed to
         another
    2: groups that seek to control a social system or activity from
       which they derive private benefit

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "vested interest":
   Black Power, Italian vote, Jewish vote, Polish Power, White Power,
   absolute interest, appurtenance, authority, benefit, birthright,
   black vote, claim, common, conjugal right, contingent interest,
   demand, divine right, droit, due, easement, equitable interest,
   equity, estate, faculty, farm interests, financial interests,
   holding, inalienable right, interest, interest group,
   labor interests, limitation, natural right, part, percentage,
   power, prerogative, prescription, pressure group,
   presumptive right, pretense, pretension, proper claim,
   property right, right, right of entry, settlement,
   special interests, special-interest group, stake,
   strict settlement, title, trust, use, vested interests,
   vested right