I can' t quit thinking that the round nature of the shaft versus the flat surface of the vane base when applied " offset" would automatically impart a helical to the adhered vane.
It does. But you can only do a straight offset just so far because the ends of the fletch couldn' t touch the shaft. A true helical wraps around the shaft and keeps the ends of the fletch in contact with the shaft. You can helical fletch an arrow a lot further than you can offset.