From what I’ve been able to tell it looks like you must complete the Alliance version of the Mechagonian Threat achievement.
In the Content Update Notes it actually says the following.
Horde players can unlock the vulpera by completing the Vol’dun level-up story line and reaching Exalted with the Voldunai. Alliance players who are Exalted with the Rustbolt Resistance and have completed the Mechagon story can embark on a quest line to create a mechagnome of their own.