Not sure why people assume this other than they are a cute race, so that means Alliance.
I think Horde have a leveling questline where the Vulpera live, but both sides will be fighting with the Vulpera to fight the Sethrak. Could see it going either way honestly. Keep in mind, the desert where the Vulpera lives isn't also a prison of sorts, a place where the Horde sends its criminals and outcasts. Could be the Vulpera are tired of the Horde's shit and want nothing to do with them, or if could be the Alliance also sends criminals/outcasts. These being the people the race isn't most associated with, I don't see them having a good view of either faction.
If they join anyone, I personally hope that Blizzard does it like Pandaren and makes them a neutral race that can pick a side.