Oh no the trad police are on duty today LOL!
Shoot what you want. To the original question both synthetic and natural will absorb water but natural will repel much better b/c of the oils. I have hunted int he rainw ith some turkey feathers and they held up nicely.
I have no problem with plastic vanes and if you plan on hunting in very wet/moist areas often I'd suggest a rest and plastic vanes. In the end we are trying to kill animals here not impress joe next door!
As for the police...if you didn't make your own hickory self bow, make your own string, widdle arrows, use a flint napped head you made yourself and hunt in your deer skin clothing well you aint traditional! Enjoy your scent lock, carbon arrows, and all natural traditional feathers...