To get a perfect vane glue line or seal, I use a dull, thin bladed knife, an old 6" Rapala fillet knife. Once you seat the vane, insert the edge of the blade between the clamp and base of the vane. Being right handed, I start on the left, moving the blade down the base of the vane with enough pressure to seat any gaps between the vane base and shaft.
This works well on right wing feathers because the little ledge on the base is facing toward you. On left wing feathers, the ledge is on the opposite side, rendering this method useless.