vower

from WordNet (r) 3.0 (2006)

vower
    n 1: someone who makes a solemn promise to do something or
         behave in a certain way; "young vowers of eternal love";
         "there are many vowers of chastity but few who observe it"

from The Collaborative International Dictionary of English v.0.48

Vower \Vow"er\, n.
   One who makes a vow. --Bale.
   [1913 Webster]