Farms, I get it -- fertile soil -- but why cities? Did people not learn from the historical records of their time, to draw lines around the boundaries of destruction and not build again within those lines?
After a few years after the eruption, nobody knew the exact location of Pompeii and Ercolanum, and also for quite some time these two areas were unhabitated and used only for farming. The emperor at the time of the eruption, Tito, tried to rebuild the cities but nobody wanted to live there anymore.
They only found out that Pompeii was there around 1500-1600. The repopulation of the area has happened during the last century, mainly because Italy was, after the second world war, in an economic boom and people built houses (and factories) without any permit. Politics was usually corrupted or "closed their eyes" for votes.
And here we are. At the point we are now, it is impossible to relocate this huge amount of people (around 2M) anywhere. The evacuation plan itself is a joke, due to the nature of the eruptions of a stratovolcano.
So, let's hope for the best, as there's nothing we can do to prepare for the worse.
Every time I hyke to the Vesuvius top, guides explain that there are other active vulcanos in the area; they are all considered active and may harm at any moment. The Phlegraean Fields are problematic because they generate a lot of bradyseism (and in some places the ground went underwater for about 10 meters!), but for now they are considered quiescent. Ischia island also is a volcano, that has erupted last time around 700 BC. Finally, there's an underwater volcano called Marsilii, that can generate a tsunami.
All these volcanos are stricly monitored.
We have the Red River Floodway (https://en.wikipedia.org/wiki/Red_River_Floodway) which has been quite successful, however, we're getting more floodwater every year and now we sometimes face severe flooding on the Assiniboine River.
This all used to be at the bottom of a giant lake and probably will go back to that at some point, but we've done a pretty good job at preventing billions in losses.