I Jeremy,
My advice is to measure and trace the distance and the vent(inside measurements) itself. Using the tracing as a guide on the decal, slice with a sharp blade only on the three sides of the vent(the bottom and two sides) starting at where the vent starts to slope inwards. To prevent the decal from tearing I would start the cut from both ends of the line of cut because the initial cut punches in rather than leaving a sharp cut at the end creating an easier give for tearing. When applying use copius amounts of water and with the help of a large soft brush slide it away from the flap meaning upwards of the bonnet/hood.
Does the color of the decal run into the inside corners of the vent, if so then a coat of the same hue needs application before decaling. If you have measured the inside of the vent accurately, you might just have enough of the decal to cover the outer lip of the vent edge. It may look like surgery but that's how I would attempt it.
The other way is to apply the whole decal over the vent minus the decalling softening fluid, let it semi dry and then slice the three edges and tease the flap into the vent...now apply the softening fluid.
Hope this helps. Good luck.
Cheers,
Richard
