It depends on the size and resources of the state. If the region around Neapolis is the heartland of this state and they don't have many settled/highly fertile lands elsewhere then Vesuvius would indeed be crippling. Aside from the literal destruction, it's also likely to cause panic and shock in the population, and might reflect extremely poorly on the rulers of this state; what worse sign could there be of the God's disfavour than destruction on this scale?
On the other hand, the actual destruction in OTL was limited to a few prosperous but small towns and didn't permanently harm the Roman Empire's economy or morale.
It's likely to be a scale than two discrete options, the result for an ATL South Italian state is going to be somewhere between these two extremes.