The 24-year-old Auckland-born threequarter has made 42 appearances for the Vikings since joining them from Castleford ahead of their return to Super League two years ago.
Widnes coach Denis Betts said: "Willie's attitude, enthusiasm and character are second to none. He is a great human being who's fantastic to have about the place."
Isa, who played in the NRL for Penrith Panthers and Melbourne Storm, added: "We've got a great bunch of players at Widnes and we're moving in the right direction.
"The club has always been an option for me and my family and I was really pleased to sign an extension to stay here for longer."