The reasoning for Vulpera being the sub race for Goblin isn’t just the rig (though that is part of it) it’s also that the sub races are faction based so an Alliance Worgen couldn’t be matched with the Horde Vulpera. Also the Vulpera came in with the Mechagnomes which lines up with Goblins & Gnomes.
Here’s an interesting fact for you guys… During WoD there was a Dev who was let go by Blizz & leaked info on upcoming new classes & races. He said Demon Hunters were going to be the next class long before Blizz made it official in Legion. The 2 new playable races he listed were Ethereals & Mo’args, which means at some point Blizz had at least discussed them as an option. This was all long before Blizz decided to start down the “sub race” plan though, so at this point it’s probably meaningless lol.
I’m still betting on sub races for Undead & Worgen who are part of the Shadowlands zones & storyline as our best clues.