Pompeii - en route to Naples?

You can store luggage at the Pompeii Scavi entrance (check the site info to be sure). Yes, you can visit Pompeii on your way - get an earl start if you will visit during the hot summer months. Time to visit Pompeii excavations seems to vary from a few hours to almost a full day.

Bag storage is free at the Pompeii site, or there's a paid option at the train station.

Good plan. As mentioned above you can leave luggage at either the station or the Pompeii escavation site.
3 or 4 hours are sufficient for a good extensive visit of the major sights of Pompeii. If you are going to Rome, just be aware that the last high speed train departs from Naples at 7:30pm.

Great, thank you all for your replies! Is a guided tour necessary or can we get by without a guide?

We did it without a guide but you should buy a good guidebook: we bought one at a shop near the entrance. The scavi covers a vast amount of ground so a good map is essential. This is a very good resource to get you started, or could be used in place of a guidebook:

http://pompeiisites.org/allegati/A%20Guide%20to%20the%20Pompeii%20Excavations.pdf

There are licensed guides onsite, not very expensive for a tour. On the other hand, if you use an audio guide or a book, you can go at your own pace.

We are also planning on taking this route. Does anyone know: From Sorrento do we buy our train ticket all the way to Naples or only to Pompei, then another ticket to Naples? We plan on staying in Pompeii for about 3 hours. Thanks.

Separate train tickets for each journey.

We did Pompeii without a guide but had a very good guidebook. The place is not well signed and you really do need a book. The advantage of a guide is that they can unlock and let you into closed houses, the brothel etc which are closed for preservation reasons. We were able to piggyback on guided tour groups -- and just walk in when they were opened since the purpose is not exclusivity but protection. But no guarantee that always works with all guides. I would probably hire a guide on site in order to facilitate that if we were to do it again. Wonderful place to visit. Carry water, sun screen, sun hat and snacks; once in you cannot leave and re-enter and the cafe in the place was closed when we were there and there is not much for sale at the snack bar. We picnicked by finding one of the very few shady spots and sat on a tree trunk. It is not set up to facilitate this.

May I make a suggestion? We used a wonderful guide for Pompeii (& Herculaneum) and his brother was our driver. You could use the brother (Gianluca Bianco) to drive you from Sorrento to Pompeii where you would meet Sasha Bianco (as your guide) and then Gianluca could drive you to Naples. Both were excellent and they made for a stress-free day for us. It would be so much easier than to take the Circumvesuviana train to Pompeii, then have to find luggage storage and then make your way to Pompeii, back to the train station, etc. I personally can't imagine visiting Pompeii for 2 hours without a guide. He really condensed the site and led us directly to the areas of most interest and I was glad not to have to find my way around on our own and fight the crowds (we visited in September and it was very crowded and hot). Pompeii is massive and I was able just to enjoy the experience and not worry about logistics, etc. Sasha really made the site come alive and helped us imagine what life was like at the time. If you want Sasha Bianco's contact info, let me know. Have a great trip!

By the way, we used a private driver for a transfer from Sorrento to the Naples Archaeological Museum (for a 2-hour tour) and then to the train station in Naples afterwards. I did not want to have to have to deal with the Circumvesuviana train, finding luggage storage in Naples, take a taxi to the museum and then make our way back to the train station, find our luggage, etc. Our driver waited for us while we toured the museum and kept our luggage in his car. It was stress-free and much more pleasant. Of course, there's a cost associated with this convenience, but hubby & I both felt it was worth it.