When i used the VHT on the tails of my first car i didnt put alot on and in about a year it was really faded.
On the car i have now i caked it on and after a year it hasnt faded very much but you can tell its not as dark.
If you put alittle on your headlights I dont think it will mess up visibility that much, however, it probably wouldnt look as good as if you put a decent amount on there.
I found that when putting it on the tails it doesnt really effect how much light gets emitted that much but it defines the light more, like if you have a square that lights up when you hit the brake it makes only the shape of the light, it doesnt really emit light out of the square, if that makes sense.