Feathers are lighter than vanes, so the arrow will leave the bow at a higher velocity than it would with vanes. They say feathers slow an arrow downrange more than vanes do, but the guys that say such things also say the vane fletched arrow won't catch up with the feather fletched arrow until it gets 40-50 yards dowrange. Frankly, I have never once seen any reliable data that proves feathers slow an arrow faster than vanes, nor have I seen any significant difference in point of impact at 70-80 yards when shooting a mixture of arrows fletched with both vanes and feathers.
So, I say MYTH BUSTED.
Vanes are waterproof. That's their only advantage. In all other areas, they are inferior to feathers.
Feathers are better at stabilizing the arrow.
Feathers are MUCH more forgiving of rest contact than vanes. You always want to eliminate fletching contact, naturally, but stuff happens.
Since they are lighter, feathers allow you to build an arrow with a higher front of center weight ratio for even better flight stability.
Feathers are not temperature sensitive, so they don't get extra stiff in cold weather or soft and flappy in hot weather.
Feathers can be damaged and still shoot quite well. A damaged vane will hiss and sound like a buzzbait in flight, and likely not shoot to the same point of aim as your undamaged ones will. Shooting groups for field archey practice, vanes get torn up and holes shot in them very often. With feathers, they get ruffled and I just smooth them back into place with a quick swipe of my hand. I've got to refletch vaned arrows 4-5 times more often than I have to with feathers, so feathers are much more durable.
Along with that, a wrinkled vane is wrinkled forever. A wrinkled feather takes only a few seconds over steam from the spout of a tea kettle to straighten out good as new. Again, feathers are more durable.
When they are new, feathers can be a bit noisy in flight, especially if they are mounted with too much helical. After a week or two of shooting and get some of the 'new' worn off, they soften up and are just as quiet as vanes. Most guys don't give feathers a chance to quiet down. So they perpetuate another myth, that feathers are noisy in flight.
There are numerous products to treat your feathers to help keep water from wreaking havoc with them. And dozens of tricks you can use to keep them dry in wet weather. They do take a bit more care than vanes in wet weather but, IMO, the benefits of feather fletching outweigh that slight inconvenience. I don't hunt in wet weather that much anyway.
Maybe if I lived in the perpetually rainy Pacific Northwest I'd be more inclined to use vanes, but I don't so I don't.
Come to think of it... When was the last time you saw a goose with rubber feathers? If feathers are waterproof enough for a goose...[8D]