Vulpera will never become an Allied Race, because they're a unique race. So far all Allied Races were reskins and I guess that's how it'll stay.
They will only ever add the Vulpera as a regular race and assuming they'll be a Horde race, then they'll also have to give to the Alliance a unique race.
Mechagnomes make a lot of sense as an Alliance Allied Race and the Horde might as well get Giblins.
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And that's why they're not adding Vulperas as an Allied Race, because that's not the point of this system. Players were always asking for subraces and Blizzard's answer to that are Allied Races: subraces that you can unlock.
If they will ever add Vulpera, then they'll be a normal race