Word from the wise i doubt if your idea of spray glue fixing such a material would work particularily in the engine bay over any sort of prolonged period if it were me i would do either
A - Obtain said soundedening materials from said scrap cars/vans and attach it in one of the following ways either by self tapper and washer or by original soundeadening clips from a walk around a scrapyard or possibly E-bay.
B - Use Flashband which is what we use in the rear to stop the panels drumming when building campers it is available from B&Q etc on rolls from the roofing isles and is used mostly for this and roofing applications self sticking and it will do the job well easily cut to size shape and applied.
Spray glues are fine for internal carpets that do not weigh very much but dampness weight and weather all have toi be considered in your application dont quite understand why you need to do it though as VW soundeadening is not ther worst ?/ does it not have any underbonnet soundeadening already fitted ? it should be on the underside of the bonnet and about half way down both inner wings ....