I'll take a crack at it. As for why they don't come, that's easy to accomplish. Bonifacius almost certainly never invited the Vandals over-he was too smart to think that inviting an entire people over into a relatively undermanned province was just asking for trouble. Rather, he likely contracted out some Vandal mercenaries in the same way Felix did with Gothic forces in his attempt to take North Africa from Bonifacius. That Genseric had an interest in getting away to Africa shouldn't be surprising-It'd been attractive to the Germanic tribes since Alaric tried it after the sack of Rome.
Anyway, Genseric had a real problem when he was trying to cross. The Suevi were raiding Vandal and Alan territory. Since he would be unable to bring everyone across on a single trip, his men waiting would be prime pickings for the Suevi. So he first gathered his forces, marched north, and promptly defeated the Suevi-so, really, just have the battle go differently, have the Suevi score a decisive victory, and Genseric's plan to bring his people over to Mauretania is now unviable.
Now as for the effects, this is a massive boon to the WRE. After North Africa was taken, the west experienced a serious money and food shortage. The coinage was being seriously debased once more, and of course, efforts that otherwise should have gone to Spain and northern Gaul were now being funnelled to retaking North Africa. Aetius is probably still going to be able to secure sole power and out maneuver Felix and Bonifacius. This means now he has free reign to focus all his efforts on reclaiming Africa and shoring up Roman control in Gaul (which does not necessarily mean attempting to destroy the Goths, mind). The Empire is now in a much more secure position, with 30 years of (relative, aside from the spat with Bonifacius) internal stability.
In the east, without the need to send Aspar with a large army to Sicily in the 430s means Attila likely has less success in raiding (the 430s invasion of Africa was called off because of Attila's raids the moment the army left). You'll probably still see him getitng large tributes of course, and he might take a go at the west, but given the western empire is wayyy outside his core area of control and is in a much stronger position than OTL, Aetius is probably even more successful in dealing with him this time around.