Hi Gail
If you put 'Pompeii' into the search function at the top right hand of the page, you will find several reviews (including mine) of this trip and loads of tips that will help you out.
The weather seems to be very much the luck of the draw. My departure was 15th September and we had a week of glorious sunshine and very hot temperatures. However, the group who went the following week had rain on most days. What I would say is that, when it's hot - it's very hot and there is little shade (especially in the ruins of Pompeii itself). Take a hat, loads of sunscreen and always carry a bottle of water. The ground is also very uneven so take really comfy walking shoes.
As for Vesuvius, you get dropped off about two thirds of the way up to the top by an off road vehicle, and then the hike the rest of the way via a zig zag cinder path which can be quite loose in places, especially if they have had recent rain. You could take walking boots but most of my group did the climb in trainers.
Have a super time
Mandy