Plinian eruptions are caused by gassy magma. Utilising satellites to monitor the changes in gas emissions and shape of Mt Vesuvius could be used of notify the authorities of a possible eruption. The authorities in turn, would then proceed to evacuate the area of all civilians in the event of an imminent eruption. This measure would result in a minimal death toll.
In the case that an imminent eruption is detected, an immediate evacuation would follow suit. The evacuation would have to be centred on removing as many civilians and as much livestock as possible, while creating as little panic and mania within the population. This measure would serve to, in the case of an eruption, reduce the death toll in people and livestock and also leave the civilians with some of their livelihood and means of earning a living to provide themselves with the necessary thing they need for rebuilding their community and economy.
Buildings in areas prone to Plinian(ash) eruptions should have steeply sloping roofs so ash does not accumulate. In the event of an unexpected eruption this would help the area surrounding a volcano prone to Plinian eruption as it could save their houses from collapsing under the weight of the accumulated ash and thus save lives. For example, if the Plinian eruption contained a lot of ash but no so much that it was lethal, all the accumulated ash could be what kills or maims the civilians. Also, this measure would reduce structural damage and save money.