Or try this -
First, color the window frame with a silver Sharpie.
Then cut a rectangle of BMF, big enough to span the entire vent window frame and burnish down with a cotton swab and toothpick.
Now, with your brand-new Xacto #11 blade, cut around the frame.
Another thing I've found is that I don't pay attention to the paint quality on the vent window frame (don't sand and polish it like the rest of the car), so make sure there are no boogers or texture in your paint on the frame. If so, remove them and hide with the Sharpie.