OK, here's the scenario:
The Vandal Fleet heading for Rome gets caught in a huge storm just short of their target. The majority of the ships are sunk or blown way off course, so that they take absolutely no part in the ensuing events.
With the survivors landing near Rome numbering only a fraction of their OTL numbers, Pope Leo throws his weight behind Emperor Petronius Maximus and urges the population to resist, or, at the very least, he keeps quiet and doesn't actively betray the Empire.
The Vandals try to take the city in one fell swoop, only to find the gates locked and the defenders fighting back.
While this is going on, the few ships Petronius has in Ostia launch a daring attack on the moored Vandal fleet and succesfully manage to burn it (in many ways simmilar to what the Vandals themselves did to the Romans OTL some years later).
Genseric dies from wounds incurred during the assault the next day. Leaderless and disorganized, the Vandals are counterattacked and defeated by a rag-tag Roman force of proffesional soldiers, mercenaries and a hastily created force of local militia and other able-bodied men from the urban mob.
What butterflies can we expect from this ?