Spoke to Mana Island Resort and it seems they are looking to start a direct boat service to and from the Island to Vuda Marina from about August 17th!
They don't have too much info on the service as yet, such as travel time (compared to the Denarau crossing) or exact cost, though they intimated that the fare would be around the $90 fiji dollar mark for an adult and half that for a child - not sure if that's one way or return though, so it could be a good deal, compared to South Seas Cruises out of Denarau, if that is the return price, or pretty much equitable, if that is the one way cost!!
Anyway, as we thought we might use Club Fiji Resort, Nadi, as our base for sightseeing locally for a couple of nights before heading over to Mana, we might end up taking the ferry over there via Denarau but return to the mainland and the airport via Vuda just for a point of difference.
Hopefully something will get posted up on this forum when Mana Island have finalised their intentions for the new transfer service, so I guess just watch this space . . .