I can only help you partially as we did not visit the volcano and only saw Pompeii itself.
But to get to Pompeii Scavi, the Circumvensuianna line (or however it is spelled) is the best option. Its not hard to find and it is not a long walk if you are walkers. Or a short cab ride. I don't know if you can catch a bus to M.V. from Pompeii Scavi or not. Ercalono is where the Herculanium Scavi siteis too..
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007