What I expected to happen was...:
Summon a Vex with no items in his right hand
What actually happened was...:
It has an Iron Sword in his right hand.
Commands to Reproduce:
(If you say it's specification. I want change this specification.)
As of 1.11-pre release 1, spawn eggs are still unable to do this, however, this is a client side glitch and reloading the vex fixes it (as server side it has an empty hand)