Speculation:

How did the Mongols go from being nomadic steppe horsemen to being the preeminent siege warfare specialists and imperial power so quickly?

They captured experts, integrated the experts into their military and social structures and rewarded them for their successes.

I imagine something similar was done by the Vandals. When a new power rolls through town and conquers you, they typically want you to output some level of economic value, and shipbuilding is most certainly valuable. The Vandals may have put their own spin on existing naval tactics too.

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>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).

The Germanic tribes didn’t genocide the former Roman subjects in their new territories. In fact civil life in much of the empire didn’t change drastically at all immediately post imperial collapse.

If a tribe conquered a territory with a maritime tradition and naval infrastructure they would inherit the naval capabilities that came with it.

Yeah, it was actually better for the North African inhabitants for the North African centre of power to be in... North Africa. They basically had been eating shit for awhile and the Romans were unable to do much, so fuck it, why not give the Vandals a chance? Hence there was little in the ways of rebellion.

I think this boils down to a fundamental popular misunderstanding by most people about who the various barbarians were and what they actually wanted. The goths and vandals were both highly integrated into the Roman system and wanted to join it rather than tear it down; they made alliances sealed with marriages to strengthen their position and often only attacked Rome when a new emperor tried to renege on those deals.

Smart answer. I have to imagine part of commandeering Carthaginian merchant marine involved capturing all of the people required to operate the vessels as well. Then, the Vandals' elite warriors essentially behave like U.S. Marines - somebody else gets them to a theater that they operate on land in.

The naval strength was already there; the Vandals just came in and ousted the Roman governor and said "hey north African navy, you work for us now!"

By this point in history the empire did not have very good cohesion, so there wasn't really a unified and effective "imperial navy" that could coordinate a retaliation; any force which might have done so was busy elsewhere. The empire was being attacked from all sides by different groups of barbarians and the Muslims. For those sailors and marines who were in North Africa when the Vandals came, the easiest and safest thing to do was to continue going about their business as if nothing had changed. "I, for one, welcome our new Germanic overlords!" If you don't piss off the new boss, you get to keep your job.

That's not speculation, it's basically what they did. The roman administration was still fully functioning there and they made use of it.

Fair. I know this is a common tactic for newer empires that take over older ones. Rather than constructing entirely new schemes of tax collection, economics, soldiering etc, most conquerors just change who is at the top while leaving the system more or less intact.

Almost everyone that conquered Persia, be they Greek, Arab, Parthian etc., all of them basically maintained the old social order.

Yeah no, this was definitely not the case with the Vandals. They crossed over the straights of Gibraltar after their confederation in Iberia was destroyed and plundered their way all the way to Carthage. Their naval tradition may have started in Spain but after capturing the major maritime port of Carthage (along with a sizable quantity of ships), they became the defacto power.

The romans in North Africa were definitely not happy with vandal rule opposed to Roman. Just read some of the ecclesiastical accounts of the vandal migration. St. Augustine was literally dying in Hippo while the vandals were en route. There were also letters sent from rome detailing the amounts of dead, and various members of the church who were killed (and nuns SA’d) by the vandals. They were of a heretical denomination of Christianity (Arians) and didn’t take kindly to those who were nicean.

There were more accepting circumstances in the migration period but not in regards to the Vandals in Africa

The Vandals ruled successfully for 100 years, in which period renovation and economic growth took place, the archaeological evidence bares this out, though the written evidence is contrary. There may be something of a conflict of interest in relying on ecclesiastical and Roman accounts with an uncritical eye as regards the Vandal occupation of AP, especially given that still then a great mass of Donatists which inhabited the region were already being actively suppressed by the very same, and found kinship in the anti Niceanism of one another. Funny also you should mention Hippo, where Donatism still flourished to a good extent. The Roman governors and administration in AP were corrupt, Bonifatius and others were disliked by the inhabitants, their ability to govern and protect the region was minimal and they were constantly getting by on the skin of their teeth. One may forget that not the whole mass of inhabitants of AP were Nicene Christians. For your perusal, I have another document I'd like to share but I'll have to wait until I'm on desktop and then we can take it further from there. You may have to wait until tomorrow for me to actually sit down, find them, translate etc, but remind me and I will get around to it.

https://www.academia.edu/27135277/The_Vandal_impact_on_North_Africa