So does the acetone paste act like a glue for the piece you want to put on?
I need to find a piece of plastic to cut and shape first. I was thinking about an empty laundry detergent jug or perhaps a window washer fluid jug. The vent is made from a kind of waxy plastic often found in large kid's toys.
The vent is most likely made from abs plastic so you need to find abs for you repair so the glue will stick. Most molded plastics will be marked as to what they're made of. Most containers like laundry and milk jugs are made from polypropylene or polyethylene.
You could buy a piece of abs sheet from Mcmaster-Carr for a few dollars and use it for the missing part. You can shape it with heat to get it close and use abs cement to attach it.