The harbour cruises from Sliema come into dockyard creek (the waterway between Vittoriosa and Senglea) and it's a nice way to get a different sort of look at both cities. I'd highly recommend one of these cruises. When in Vittoriosa a number of boatmen who loiter about will invite you for a "harbour cruise". I never took one of those cruises so I don't know how good or bad they are.
There are frequent, regular and cheap busses to Vittoriosa from Valletta. We got the bus that was heading to Kalkara and got off beside Dock No 1, the now disused dock at the head of the Vittoriosa waterfront.
If you're lucky enough to own a yacht or a cruiser then you can come to Vittoriosa that way. The marina is full of lovely (and very expensive looking) boats. This must be a fantastic way to visit the island.