A.T.
If the night air is cool, take him outside in it. Croup starts as a cold, and progresses sometimes into more (croup). Keep him cool, especially at night, b/c the hotter they get, the more they cough.
My son has had it 3 times now, and the last time had steroids and breathing treatments. (he is two). It is scary, but the worst is definitely over, I think.
I don't know about Benadryl; it probably won't do much. Triaminic yellow is good for stuffy noses and has guafisen so it helps with the deep cough.
Also, you can put him in the bathroom with the shower running and steaming it up (sit in the steam), and a cool humidifier works well too.
I don't know about vicks either...
Good luck!