In 515 Carthage was home to between 10,000 to 30,000 Vandal conquerors. By that time this tribal group had been settled in the city for about the last 80 years. They lived as wealthy land owners amid a highly industrious North African (Roman) population of several million. They may not have been all that Roman in manner - since while walking through Carthage in the year 515 a visitor would have seen little of that city's grand classical architecture. Mostly unmaintained by the Vandals, the grand buildings and boulevards had long been swamped by little fill-in houses and bustling merchant booths. The wider trappings of Roman civic life had been left to crumble and grow weedy for more than a century. So one has to suspect that the Vandals cared little for many of the finer trappings of Roman life.
In such a stable-but-decadent situation it seems possible that some Vandal inter-marriage had occurred with Roman estate holders and rich merchants, and perhaps even with the growing number of artisan industrialists. If this happened I would imagine that it was most likely among the lower echelons of the ruling class of Vandals and/or those living outside of Carthage itself - those who had cause to learn to speak something other than Gothic.
Yet we know that each ethnic and religious group had long been clearly recognised within the state for what it was. Such clear distinctions could only be maintained over 80 years if Vandal wives had been found. Most importantly for the Vandals, these would have been Arians in religion like their husbands and have spoken Gothic. Carthage had a strong navy and close connections to the wider Vandal diaspora, so bringing over brides from the homeland would have been quite feasible throughout their time in Carthage.
Native Berbers would have been unlikely marriage partners for Vandals. By the year 515 the weakening but-still-rich Carthage was coming under increasingly strong military pressure from native Berber tribes, a group who took the still somewhat-important trading port of Leptis Magna eight years later in 523. In such a situation any slight intermingling with a local Berber population, if it happened, was probably merely in the form of a very sparse number of tavern bastards.
Given all the above, it seem that the Vandals of Carthage were likely to have overall retained their distinctive Middle European looks throughout the period. The Byzantine historian Procopius wrote of the Vandals that: "they all have white bodies and fair hair, and are tall and handsome to look upon, and they use the same laws and practise a common religion. For they are all of the Arian faith, and have one language called Gothic ... This people used to dwell above the Ister River [River Danube, Romania] from of old" (Procopius, History of the Wars).
The year 515 was nine years into the reign of the weak king Thrasamund (reigning in Carthage from 496–523 AD), but he also had eight years to go. He appears to have refrained from persecuting Catholics, in order to placate Byzantium. So it seems to me that Carthage was rather stable in 515 - a stability perhaps evidenced by many reports of of the sorts of industry and merchant ventures that spring up in peacetime.
The Byzantines drove the Vandals out of Carthage in a short war in 533–4.
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