Is Sorrento a nice choice to stay?
Yes, especially as March is off season on the coast. There will be more hotels and restaurants open in Sorrento than in some of the smaller villages. The ferries also won't be servicing a lot of those little villages around the coast in March so having bus and train service to/from Sorrento and Naples (and maybe limited ferries to some points as well) will be an advantage. But see my note at the end of this post...
Pompeii one day and Herculaneum the next?
Sure, you could do that but could also do Herculaneum on your way to or from Sorrento.
Archaeological museum in Naples worth it (we think so, desire
opinions) and this would be done w/ Pompeii or Herculaneum?
Yes, well worth it but I wouldn't combine it with Pompeii as that site is HUGE: much larger than most visitors expect it to be! If at all possible, see the museum AFTER you've seen Pompeii so the artifacts have some sort of context.
Should we train in to the sites and purchase a guide there or purchase
a viator guided tour for transport from hotel and site OR train in and
use Rick Steve's guide?
Book a Mondo tour as recommended! Viator does not operate their own tours but are a 3rd-party booking site for a variety of independent agencies.
We ruled out Vesuvius trek uphill. Mistake?
No, not a mistake. IMHO, seeing Vesuvius rising above one of the cities it destroyed (Pompeii) is pretty darn poignant. Just imagine what it may have been like, from your standing point, to have witnessed the enormous column of ash and debris the volcano spewed before superheated pyroclastic surges erased all traces of life.
So here's a question? I'm hearing more interest from you in the Pompeii and Herculaneum excavations + the Naples Archeological museum than anything along the more southern coast. You might consider just staying in Naples versus Sorrento?