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