The Vision class is a bit confusing because even though they call it a single class of ships, it really is three pairs of sister ships and thus more correctly three sub-classes. Plus, when Enchantment was stretched it broke one pair up and so you have 4 sub-classes!
If that isn't enough confusion, the class is named for the last ship built and not the first! Vision Of The Seas was the last of the six ships to enter service.
The three 'sub-classes' are: Legend and Splendour, Grandeur and Enchantment (but not really anymore), and the Rhapsody and Vision. The first two are the most different, and the last four are the closest alike.