An interesting passage in Will Durant’s The Story of Civilization IV: The Age of Faith,: (1950), p.30. The author summarises an argument by Salvian of Marseilles, ca. 450 AD, De Gubernatione Dei, “On the Government of God”.
The Vandal chieftain Gaiseric, on capturing Christian Carthage, was shocked to find a brothel at almost every comer; he closed these dens, and gave the prostitutes a choice between marriage and banishment. …24
An interesting statement, as part of Salvian’s theme on how much better the Germans were morally than the Romans. But what does Salvian actually say?
The footnote, sadly, is to a number of sentences:
This makes verification difficult. Google makes it harder by hiding the fact that some of these books are freely available at the Internet Archive, in order to sell copies. The Agricola isn’t relevant; the Bury page appears to be wrong. Fortunately all these sources are old, and so out of copyright, and it is Boissier, p.420, who appears to be Durant’s actual source. He even thoughtfully gives a proper reference:
Surtout ils sont chastes; c’est une honte chez les Goths d’être un débauché; chez les Romains, c’est un honneur. Le premier soin de Genséric, quand il eut pris Carthage, fut de fermer les lieux infâmes, qui se trouvaient à tous lès coins de rue, et d’éloigner ou de marier les courtisanes, et c’est à un barbare que la ville de saint Augustin doit d’avoir été purifiée.
Above all, they are chaste: it is a disgrace among the Goths to be a debauchee; among the Romans, it is an honour. The first care of Genseric, when he had taken Carthage, was to close the infamous places, which were on every street corner, and to exile or marry off the prostitutes, and it is by a barbarian that the city of St. Augustine had to be purified.
 VII, 20, 84.
I could not find in Boissier any indication of what edition he used. But I consulted the Sanford translation (1930), and found nothing relevant in book 7, ch. 20. I’m not sure what “84” indicates. But chapter divisions vary among editions. In Sanford, in ch. 22, on p.219, I found this:
22. … I said that the cities of Africa were full of monstrous vices, and especially the queen and mistress of them all, but that the Vandals were not polluted. … For they have removed from every part of Africa the vice of effeminacy, they have even abhorred intercourse with harlots, and have not only shunned or done away with it for the time being, but have made it absolutely cease to exist. …, they removed unchastity while preserving the unchaste; they did not kill the unfortunate women, lest they should stain their prevention of vice with cruelty …. They ordered and compelled all prostitutes to marry; they transformed harlots into wives, … In this, indeed, provision was made not only that women who could not live without husbands should have them, but also that through their domestic guardians those who did not know how to protect themselves should be safe. While the marriage bond constantly bound them, even if the customary unchastity of their former lives enticed them to sin, their husbands’ guardianship should keep them from going astray.
Which is all well and good, but does not justify the claims, that there was a brothel on every street corner, and that closing the brothels was the first act of Genseric on taking Carthage. I was unable to find these elsewhere in Salvian.