One full day for Pompeii and .. ?

We were able to spend the morning at Pompeii and the afternoon at Herculaneum without being overly rushed because they are relatively close together. Of course one could spend all day at either one. It just depends on how deeply you want to experience the site(s) as opposed to just doing a once over lightly.

Zonehiker, I might suggest Pompeii and the archeological museum in Naples, where many treasures from that excavation and Herculaneum reside. The museum would be a good educational enhancement of the Pompeii experience. Yes, you can use the battered but very inexpensive local Circumvesuviana commuter train for travel between Sorrento, the scavi and Naples.

Just don't do it on a Tuesday when the museum is closed. :O)
https://mann-napoli.it/en/home-english/

Personally, I'd skip Vesuvius. You can see it clearly looming over the ruins and other locations around the region, including in Sorrento from perches around the Gulf of Naples.

It depends on level of interest, but you'd have no problem spending an entire day at Pompeii; in April we spent 10 hours over 2 days plus a trip one station north to see the Villa of Oplontis; I highly recommend adding that to the day. End the day at the Archeological Museum in Naples to tie it all together.

We took 3 days to see ancient sights in the Pompeii area, commuting from our lodging in Sorrento. With a single day, and it sounds like you’re not planning on seeing every possible (but fascinating) sight, definitely spend time at Pompeii. You’ll get a spectacular view of Vesuvius in the distance, towering over the ruins. I’ve been atop many mountains, so maybe it’s not a once-in-a-lifetime experience for me, but I’d suggest not taking the time to get up onto a volcanic surface to look down on the area below.

Instead, plan on adequate time for the complete Archeological Museum in Naples. Most of the artifacts and art from the Pompeii ruins are in the museum, not at the ancient site. Missing the museum would be missing a lot. While you’re in Naples, wrap up your Italy trip with authentic Neapolitan pizza. Compare it with some other pizza you might have had over the previous 3 weeks :-)

https://community.ricksteves.com/travel-forum/italy/pompeii-and-vesuvius-69e8366c-1e75-4a14-a0bc-0677269492d1

I can't tell if you like art museums or not. A glance at Naples, and dinner (?) in Naples is worthwhile, if you only spend two or three hours inside Pompeii. I recommend Pompeii and Herculaneum, if you never go to museums at home. There are replicas of many of the sculptures in the museum, in their original locations in Pompeii. However, visibility of the fresco replicas in Pompeii is not very good. Pompeii is absolutely HUGE, but I believe you've made it clear you don't need to feel you saw every corner. Get a book with a walking route, like our host's here, Rick.

Staying in Sorento and are thinking of taking the train.

Just in case you haven't found it yet, here's the schedule and some other info on the Circumvesuviana:
https://www.sorrentoinsider.com/en/naples-to-sorrento-train-schedule

To get the full Pompeii experience, I highly recommend contacting gaetano manfredi for a guide in Pompeii. We used him as our guide and the trip to Pompeii was incredible

IMO the Museum is not to be missed.

I would hit up Pompeii early in the day, get one of the best pizzas in the world for lunch at Da Michele or Gino E Toto Sorbillo, then spend a couple of hours in the Archaelogical Museum.

It's not just artifacts from Pompeii there FYI, there's also an excellent collection of statues that rivals anything in Rome or Athens

Thank you all for taking the time to answer! I think we will go with taking the train ( great info thank you for sharing ) and heading to Pompeii first thing! Then we will hit one of the pizza places suggested ( another great help narrowing down to those two!) and then to the museum.
I agree, days could be spent taking in all that Pompeii and the surrounding areas have to offer. I feel very good about the day we will have ( agree also a guide will be such an enhancement, I will reach out to the suggested guide - thank you).
Our time in Italy will be full of museums, historical sites and wandering through towns and cities. Can not wait!
Thank you for helping me decide the best way to see Pompeii with the time we have!