>But how could the Vandal military go from a roaming war band/army through continental Europe to becoming a hegemonic entity in the western Mediterranean, with no prior experience or knowledge in ship building or naval logistics?

Let's not forget the Romans in the beginning weren't much of a seapower either, up until the Punic Wars. In the case of the Vandals, their earlier conquests and Roman concessions in Hispania (Spain) gain them access to maritime ports and naval bases such as Carthago Nova (Cartagena). From there it was only a matter of incorporating local naval experts and ship crews into their military. Even before capturing Carthage, they were already conducting naval activities in and around the Balearic islands from theses Spanish bases.

Absorbing defeated enemy troops and specialists was not an uncommon thing to do in those days, and the Vandals and other Germanic groups weren't the mindless brutes that late-Roman writers will have you believe (especially given that they recruited these same folks into their legions and even gave them high-ranking military positions).