You made it work, that’s the main thing!
I made my suggestion, the whole procedure. Fixing the missing face was part of that process, just add the edges that were needed to make the face. You can make solids if you want. That’s another issue, but they are not needed to do what you asked: to cut the material at the vertical angle.
Simple: I would not cut it like this. In reality no one ever will. I’d just extend the vent up and leave it at the top. Maybe you want that vertical slice with all the faces to demonstrate in some graphic for apprentices, I don’t know. But it doesn’t have to do with construction practices IMO.
It’s sometimes easier to keep components intact as possible without a lot of operations that will mess with their geometry. The vents are always the same, some are longer than others. They don’t have to be sliced to the angle of the roof. Just make it longer. Also, as I said, I wouldn’t use “Intersect” on the insulation. it is such a simple shape to adjust with the move tool.
(I was just thinking of this, making a metal window frame for my own model. I was fussing with follow me to run it around the window, and the complexity of the shape was making subsequent modifications really difficult. I went back to just using extrusions to make each side of the frame, because that is how they are fabricated anyway–they don’t run “follow-me” in the factory, and it just complicates my modeling in this case. Now my extrusions don’t have to be messed with so much at every joint.)
Selecting the waste to remove. I explained that best I could: right to left selection- but don’t touch anything you want to keep such as the cut face edges. Then catch any leftovers with eraser tool. It goes quickly once you’ve done it a few times.