By car: You take the Highway E6 from Trondheim to Otta, then you cross the Sognefjell (highway R55) to Sogndal and Hella. Ferry from Hella to Vangsnes.
By bus: go by the express bus between Trondheim and Bergen, but leave the bus at Lavik, then enter a local bus to Dragsvik/Sogndal. And go by the ferry to Vangsnes.