It's weird you ask that, but this is the right time of year to find out as with all the condensation that forms on your car (including up the bonnet) - all you need to do is take it for a short drive. The condensation will evaporate from the areas where there are no support structures, and remain above the "struts" after a short run - that'll give you an idea as to where you cna fit them. Alternatively, just open the bonnet and have a look at the underside.
As for the vents, they are open angled backwards, therefore air flowing over the bonnet will drag air out of the vents, thus removing most of the hot air from the engine bay. Therefore whilst moving, the engine bay will remain cooler (better for the engine) and also the air filter will not be sucking in as much hot air as it will be with the vent's.
However, when sat stationary the air in the engine bay will still heat up and only "seep" out of the vents - so the filter will suck up hotter air, reducing the fuelling of the engine, and thus, the performance when you go to pull away. It could take about 30 seconds for the engine to return to a propper "cold air" operating condition (i.e. better performance) as all the pipes and the filter would be heat soaked (how much depends on how long you were stationary for).
Hope that helps clear a few things up