Narrative films should still follow the basics of Introduce Problem, Complicate Problem, Resolve Problem. If there's no goal then there's no story. A scene or sequence and not necessarily a story. A vignette I think is just a scene or sequence.
As to length: as long as it needs to be to tell a compelling story, and no longer. You can do a story in 7 minutes or less (like this short subject I made 2 years ago...CLICKY
). That length need not be a vignette.