Is it practical to visit Pompeii - to visit the ruins and climb Mt Vesuvius, - as a day trip from Naples?
It is possible but probably not practical
It’s a lot for same day
I did it about 10 years ago as a day trip from Sorrento.
I was on a tour, and the tour bus took us to Pompeii early in the morning. Then, around lunchtime, four of us decided to break off from the tour group and do Vesuvius in the afternoon. The Scavi train station is right next to Pompeii, and we arranged transportation to Vesuvius from a ticket office not far from the station. A bus took us to the entrance to the national park, where we boarded a massive off-road vehicle that took us to a parking area not far from the summit. We hiked maybe another 20-30 minutes to get to the summit. We took the same route back to the Scavi station, where we caught a train back to Sorrento. My recollection is that we were back in Sorrento before dark and in time for dinner.