After lurking in a few Late Antiquity threads, I've seen the theory tossed around that the loss of Africa to the Vandals was the real nail in the coffin for the Western Romans due to the significant loss of revenue. This leads me to two questions:
Did Romans at the time recognize the severity of this loss? Not in the sense of predicting the fall of the Empire, but recognizing the gravity of the loss of not only a significant revenue-generating area, but also just a generally core part of the Republic as well
Did any of the myriad of efforts to retake the region stand a realistic chance of retaking the region? (Battle of Cape Bon in 468, Majorian's planned conquests, others I'm sure I'm not familiar with, etc)
On 1. I think by that point (c.429) it was clear the West was already in terminal decline. Rome's sack in 410 by the Visigoths was recognized by the Romans as an unprecedented disaster i.e. Augustine wrote The City of God against the Pagans in response. The Romans had lost Britain in 411 after Constantine III died and Gaul was a mess. The loss of North Africa was just the latest of a long line of disasters in the half century after Theodosius I.
Clearly Sicily and North Africa were important grain producing regions (Sicily contributed a lot to Ravenna's prosperity in the centuries that followed)- but I think the symbolic sack of Rome probably appeared more terrifying to the senators and emperor (even though he was in Ravenna at the time) than anything else.
On 2. Justinian I managed to reconquer North Africa just over a century after it fell in c.533 with c. 15,000 men (10,000 infantry and c.5,000 horse) in the Eastern army, yet the 468 Cape Bon effort with a combined East-West army of c.30,000+ soldiers failed. Clearly something went wrong- perhaps it was the fact Cape Bon was a naval engagement and the Romans had always had a much better army than navy. Or perhaps it was the fact that Justinian as sole emperor was able to co-ordinate his forces more effectively than the joint West-East force at Cape Bon. Or that the Vandals were internally divided at the time of the later invasion.
What I would say is that Cape Bon was probably the best chance they had during this time- in my view the Western Empire alone was barely strong enough to hold its existing territory, let alone take new territory. The East, which had a longer history of urbanization, had less barbarian influx and had a nearly unconquerable capital. In the c.75 years to 474, it had 4 emperors, the West had 14. For any Western Emperor such as Majorian who considered acting alone, there must always have been a risk in the back of their minds of a usurper collaborating with the Vandals against them, which meant they marched with one hand tied behind their back.