Awe heck,,,what are you complaining about ? It takes more time to hack away at 10" versus a little 6 incher,,, Its called getting your money's worth. lmao
I would just call them back and have them fix it right,,,Your paying for a job done correctly.Here is what I would do.
Instead of trying to fix it yourself call them back and have them ;
1.Purchase the correct vent kit.
2.Remove the installed vent and shingles and felt exposing the decking.
3.Install new decking.
4.Follow manufacture directions for placement and installation.
5.Cut a new hole,,size as stated in the directions by the manufacture.
6.Install new shingles and seal the vent.
**Most IMPORTANT** At anytime you see any head scratching by your installer have them read the installation guidelines by the manufacture**
I like your Most IMPORTANT PART. Short story, I was installing a heat pump thermostat years ago (10 wire) I had the wiring diagram on the wall and kept looking at it when the owner walked in and said Uho! you look like you don't know what your doing. Reply I do know what I am doing , it's the guys who don't use the diagrams that don't know what their doing.
No Warranty Means- If it breaks in half you get to keep both halves.