Nah. If a species hangs out in the Vale long enough to forge a symbiotic relationship with whatever makes the flesh-eating gas down there, then its wings would have atrophied and vanished a long time ago.I feel like Vaal was corrupted by the Vale. It used to be a beautiful dragon but it spent too much time in the effluvia and now it's basically fucked.
Maybe it was a beautiful dragon with a rider that it loved. The rider died in the Vale, so Vaal stayed there like a dog sleeping on its master's grave. That's why it only remembers it can fly when you mount it.
I made a lore!
We see species that lose the ability to fly IRL in way less time than what that would take.