According to the rocket builders, the most efficient vane is rather tall and short. The most efficient cross-section is an airfoil because the job of the vane is to provide a counteracting lift. An airfoil does this very efficiently as long as it isn’t misaligned so far that it stalls.
But, with natural materials, this isn’t so easy to do, especially using feathers. I think the Turkish Flight arrow fletch profile is a pretty good compromise where feathers are concerned. It would require a very stiff feather to go much taller and shorter without fluttering.
Alan