Shot a deer with a crossbow. I recovered the bolt, but one of the fletchings was about half gone. Will this significantly affect the flight of the arrow?

Yes. It will more than likely corkscrew.

yes, you just messed with the air flow and steering ability of the fletch. Fix it.

I agree. and one fletch is easy to fix. Simply apply some super glue to the vane and hold it in place until set.