Your itinerary is fine. Per your question about transportation, I'm a bit curious how you're planning on reaching/departing Sorrento to begin with?
Daylight will be short in early March and you could see some rain. I would keep your Pompeii and Capri days flexible as while the scavi wouldn't be ideal in wet weather, it would probably be better than Capri. Understand as well that it'll be "off" season in the Sorrento/Amalfi region so many shops and restaurants on Capri (and some in Sorrento) will be closed.
Unless the weather is too stormy to do so safely, ferries run all year between Sorrento and Capri (islanders use them for transport to the mainland) so no issue there. Winter schedules are not up yet but check this page later for timing:
As noted above, take the Circumvesuviana commuter train to/from Pompeii: very easy, very cheap, and only takes abt. 1/2 an hour. It's an unlovely, bare-bones thing but gets the job done and also runs to Naples. The station you'll get off at is Pompei Scavi Villa dei Misteri, right in front of the main entrance. Pompeii scavi is very large and can easily absorb the lion's share of a day if deeply interested. Guides can be hired at the entrance (and you can spend more time exploring when your tour is done), or you can also purchase a guidebook and do it on your own. In addition to the website previously linked, here's the official version, although some of the info might be the same.
On Capri, the best thing to do is just hike around the island and enjoy the scenery (skip the Blue Grotto) Suggested treks are here (link below) and you do NOT need a tour for any of it. Get a map at the tourist info center and off you go. Oh, the chairlift up Mt. Solaro (from Anacapri) is fun.
Sorrento is not all that big so seeing the "sights" doesn't take an entire day, A suggested self-guided walking tour here:
Villa Comunale, mentioned in the walkabout itinerary, is a nice spot at dusk to watch the lights twinkle on around the Gulf of Naples. :O)
Edited to add: Ferries will not be running between Sorrento and Positano or Amalfi in early March.