Frayed duct vent - can it still be used?
You may not WANT to, but in some decades of dealing with that junk, my approach now when I come cross the junk is to replace it with solid metal pipe and solid metal joints, and be done with it. So that's what I'd strongly suggest doing, like it or not. it's low quality, reduces airflow, and is generally horrid.

As for re-using it, which I recommend against - wrap a strip of duct tape around it past any damage to make a nice straight guideline for cutting, and cut along it. Use a nice sharp set of aircraft snips. Cut, do not tear.



If then too short, add a coupling or use some solid vent pipe to make up the difference.