remove air vents

My guess would be that the damage has already been done if the glue is anything like properly bonded. The problem with trying to soften the glue is going to be actually getting to it. Assuming it absolutely won't budge I'd have thought cutting/shaving as much of the vent away as close to the bonnet as possible would be a start then carefully cutting/shaving through it at one point to get an edge that you can start trying to lift.

Go to the scrappy and get a replacement then stick the old one on ebay to recoup some of the cost.

agreed. it's new bonnet time :o)

Either that or be prepared to sand/smooth and spray the "vented" one.