I can tell you don't have a true rotary vise. If so you could hold the feather in your fingers and turn the fly, which is what I do 99% of the time.
One thing not mentioned here among the excellent ideas is the universal hackle plier. It has a uni-joint to allow the jaws to stay aligned with the hackle's 'grain' thus removing axial torsion from the stem and reducing stress, breakage dramatically - not to mention keeping the barbules pointing outward (perpendicularly) from the hook shaft more easily.
With a uni-plier the concern becomes whether it's gripping the feather hard enough, some are a little whimpy in terms of holding power. Buy the kind you can adjust (bend) to increase clamping power if needed.
Since buying the true rotary, I have probably used the universal hackle plier once and it was only for nostalgic reasons.
Your results may vary,