[quote=Marrow]
they could try to convince to people to evacuate, that would seriously affect the worlds population by both number and evolution, not even to mention the art and knowledge that will be either lost or gained
[/quote]

They could try to convince a whole town to evacuate but people would think that person warning them was crazy and there wasn't exactly the technology back then to prove that it was going to explode. Plus the tremors that occur before the explosion would have been seen as a sign from the gods and not a sign of eruption and the Romans were very stubborn with their believes.