Why can Doduo and Dodrio learn Fly? Why can't Scyther and Yanmega fly? Game Freak is inconsistent with learners of Fly, and it seems this is the case with Vullaby. It was entirely Game Freak's decision to allow it to learn Fly, so you can really only ask them for a definite answer. Ultra Sun's dex entry says:
Vullaby’s wings are short, so it can’t fly yet, but it jumps around, dreaming of wide open skies.
It's possible that these jumps are considered as "flying", even if it isn't really. The Pokedex has also presented false information, or information that looks impossible (e.g. dex entries of Spoink, Magcargo, and Kadabra). Perhaps Vullaby can fly for short distances, but whoever writes the Pokedex believes it cannot.
Of course, there's no definite answer, this is all speculation.
Hope I helped!