The reason that Geiseric took Africa when he did was because the Vandals were outnumbered by both the Romans and Visigoths in Spain. There were very few Vandals at the fighting age, and after twenty-seven years in the Western Empire, they were losing. They were basically confined by there enemies to the south of the peninsula, which was just as well for Geiseric, whom was convinced, justly so, that seamanship was the key to his people's survival. Despite their establishing a military pact with the Visigoths, the Western Romans were militarily overstretched, so certain parts of their territory, were either under-garrisoned (like Spain and Africa) or had been virtually abandoned (like Britain). Since he had spent years either shipbuilding, or capturing ships, and Africa was not only lightly defended, but out of reach of his enemies, and also, a well known agricultural region. It was the perfect place to flee to. The plea for help by Governor Bonifacius was the pretext for the Vandal invasion.
Unless Geiseric died young, or the Roman authorities had allowed the Vandals settlement within the Empire when they requested it in 406 C.E, perhaps then they would have kept Africa. If it weren't for the Vandal invasion, then the more patriotic Romans during the time of Odoacer or the Ostrogoth Theodoric, would have had the refuge they needed.