RE: Why are feathers bad in the rain?
Bulzeye, feathers (unless you're a duck) are adversely affected by water. They lay down and won't hold their shape. Now why synthetic (man-made) feathers do the same thing is beyond me. But they do the same. They'll lay down flat on the shaft and give you no guidance assistance. Weatherproofing them is a must if you hunt in damp climates.
Feathers are more forgiving if they touch a limb or something while in flight. They are also (initially) faster flying. At longer ranges (40+) they slow more rapidly than vanes do because of the drag they create. Feathers are also noisier than one piece vanes.
Vanes have the advantage of being weather proof, durable, and quiet.
It's a personal preference and I've shot both. I prefer vanes.