The Collaborative International Dictionary of English v.0.48:

Vigesimal \Vi*ges"i*mal\, a. [L. vigesimus twentieth, from
   viginti twenty.]
   Twentieth; divided into, or consisting of, twenties or twenty
   parts. --Tylor.
   [1913 Webster]

WordNet (r) 3.0 (2006):

vigesimal
    adj 1: relating to or based on the number twenty