Meanwhile, in the other Italy.... on ringing the Circumvesuviana call centre just now, the answer to my "what's happening on that Sunday?" question was that - as yet - it's too early to say whether the service would run for the entire day, or just half !
And on asking when that information would be available, I was told it would be "towards the end of March".....
I'd suggest you continue to check their site, where - eventually - the Easter details should appear!
The English pages start here: http://www.vesuviana.it/web/en
Better still, contact them yourself before finally deciding how to travel - there's an Italian free phone number:
http://www.vesuviana.it/web/en/Contatti
Just in case though, get yourself a copy of the Alilauro hydrofoil schedule - whose current version is available from the PDF link here:
http://www.alilauro.it/index.php?lingua=english
The one on offer at present is valid until 31st March - so wait perhaps until the next version goes up?
Peter