No, there’s no real difference between the Venice Water Bus and Vaporetti—they're the same thing!
"Vaporetto" (plural: Vaporetti) is the local Italian term, meaning "little steamer," referring to the steam-powered boats originally used in the 19th century.
"Venice Water Bus" is just the English equivalent, used to describe the city's public ferry system for tourists.
Both refer to the network of boats operated by ACTV that serve Venice and its surrounding islands. So whether you call it the Venice Water Bus or Vaporetto, you're talking about the same essential transport system!