Real proof? No. But there is a lot of suggestive proof.
- The In-Game cutscene race condition comment (showed Vulpera, file is only used for playable races otherwise)
- Animations that would otherwise serve no purpose being added to the Vulpera model (Heart of Azeroth animation, for instance)
- Consistent hints keeping the Vulpera linked up with the story (Kiro at Rastakhan's funeral, the Zandalari Unlock chain referencing the Vulpera as Zandalar allies, etc)
- Vulpera Flame Keeper during Fire Festival (pretty sure that's the only non-playable Flame Keeper specific for a faction aside from one in Stormheim, but can't swear to it)
Again, nothing is a complete lock until Blizz announces it, but the evidence seems to be mounting up more and more.