Hello
I haven't been on this particular trip, but I have been up Mount Vesuvius before.
Vehicles can only go so far up the volcano, if you want to reach the summit then you will have to walk the rest of the way yourself. As you can imagine the terrain is uneven, and there is a risk of sliding down on your bum on the way back down - just don't go in your sandals..You can hire walking sticks to aid you if you want.
You would need the best part of an hour to walk up, see the crater, make your down again and check out the lone gift shop. You won't find anywhere proper to stop, sit and wait except in the mini bus.
The view and the crater is worth walking up there for me personally. But I don't think it would be worthwhile going if your not interested in walking up to the summit as there isn't really any thing else to do there. You would be better relaxing for the day and then treating yourself to something nice in Capri.
Chris