The Collaborative International Dictionary of English v.0.48:

Vexer \Vex"er\, n.
   One who vexes or troubles.
   [1913 Webster]

WordNet (r) 3.0 (2006):

vexer
    n 1: someone given to teasing (as by mocking or stirring
         curiosity) [syn: tease, teaser, annoyer, vexer]