The only reason feathers are faster than vanes for a given length is the fact that feathers are much lighter......
Check out this ballistic calculator and you can plug in different lengths/heights of vanes and feathers and see for yourself the affect that drag has on both styles:

http://home.att.net/~sajackson/ballistics.html

Keep in mind too that feathers or vanes and their varying weight has an effect on arrow spine.For any given head weight, adding weight to the REAR of your arrows will in effect stiffen your arrow's spine while lightening the back end will soften it. This helps to know if your arrows are borderline spine wise for your set-up and you are having some tuning difficulties.

Almost forgot to mention.....I shoot 3" Right helical feathers off my A/C/C 3-60's

I hunt with only feathers, wouldn't think of useing anything else.

vanes for me as well

I like feathers better myself.

I shoot feathers. They seem to be more forgiving.

I ONLY USE VAINS NEVER TRIED FEATHERS[:-]