I admit the question of what the vulpera provide their faction has plagued me, too, compared to all of the other allied races providing obvious boons:
Void elves: Void magic
Nightborne: Arcane constructs and portal networks
Highmountain: Aerial support
Kul Tirans and Zandalari: Naval support (at least before the Battle of Dazar’alor)
Lightforged, Dark Irons, Mag’har, and mechagnomes: Advanced tech
I guess vulpera just provide warm bodies plus scavengers and trade networks.