I mean for my reply to be neutral on where to train from. The Circumvesuviana stop is at the Porta Marina entrance, and the closest entrance to the closest Trenitalia stop (there are 3 in modern Pompeii city) is Amfiteatro. That is the key to looking right at the situation in Google Maps, if you care to. You can even enter a walking route and read off the distance.
Speaking of Google, while it is not perfect on mass transit, it can give clues. Cetara looks really attractive, and Google Maps suggests that there is a bus between Cetara and Salerno, the number 5020 or 5120. That is the key to looking at the SITA (transit agency site) for the schedules of those busses. But they might only go to Vietri sul Mare, and another bus to Salerno??? You have to learn which newsstands or tobacconists sell the tickets in Cetara and/or Salerno. Look for a ticket validating machine on the bus, I don't remember that from our five nights in Sorrento.
The search box top left can give you a lot of reports on the Circumvesuviana, which tends to be very, extremely, seriously, staggeringly crowded. And hot. But it's really cheap. I don't know how much trouble it is to buy tickets for the local (not the express from Salerno to Naples, of course) Trenitalia train. Maybe there are ticket machines in the Salerno station. We took the Circumvesuviana from Naples to Sorrento, but the experience helped us decide to pay 90 Euros for a Mercedes back to the Naples train station! That and the 15 Euro taxi ride in Sorrento to the hotel.
I personally would shy away from a daytrip that requires a bus (or two) ride followed by a train ride followed by a cab ride, but it's a matter of taste and budget. You need to also add up the number of people and compare with a (150????? Euro) cab from Cetara to Pompeii. You can't afford to pay the driver 5 hours to wait for you!!!! This is not like a Uber ride home from a bar after work at home.
If you are staying at a real hotel in Cetara, maybe you can get some practical advice (at least about Salerno) from them. Another good reason not to stay in an AirBnB. (Personal prejudice, plus the damage they do to the local housing market.)