For years, I would just cut a square hole with my knife and hope the vent cover hid the edges. Last month on a job in Springfield, the homeowner had these huge, ornate brass vents that left gaps. My boss showed me to trace the vent shape onto paper first, transfer it to the carpet, and cut from the back. It added maybe ten minutes but the fit was perfect, no visible raw edges. Anyone else have a better trick for tricky vent cuts?
My buddy Dave tried to freehand a cut for a bathroom sink pipe last year. He ended up with a gap so wide you could fit a pencil through it. Had to use a whole tube of caulk to fix it, and it still looks lumpy. Your story about the octagon vent, @jordant69, gives me flashbacks to his whole mess. That's exactly why I tell him to just take the extra minute to trace the thing. It saves so much trouble later.