The Collaborative International Dictionary of English v.0.48:

Vincible \Vin"ci*ble\, a. [L. vincibilis, fr. vincere to
   vanquish, conquer: cf. F. vincible. See Victor.]
   Capable of being overcome or subdued; conquerable. "He, not
   vincible in spirit . . . drew his sword." --Hayward.
   "Vincible by human aid." --Paley.
   [1913 Webster]

   Vincible ignorance (Theol.), ignorance within the
      individual's control and for which, therefore, he is
      responsible before God.
      [1913 Webster]

WordNet (r) 3.0 (2006):

vincible
    adj 1: susceptible to being defeated [syn: beatable,
           vanquishable, vincible]