The Vandal Kingdom does not fall to Justinian. How does it develop and could it have resisted the Arab invasion?
Well, the only way to keep the Vandal Kingdom from falling is to divert the interest of the Byzantine Empire. Once Justinian decides he wants to conquer Roman North Africa, they're going to have a next-to-impossible time stopping him.
Perhaps they would accept tributary terms? The Vandals agree to provide troops for the invasion of Italy as well as monetary tribute. The Byzantines certainly had no particular objection to deals like that, and once the Byzantines withdraw once again from the West, the Vandals can regain their independence.
Cheers,
Ganesha