Assuming we're talking about 4" fletch, you'll see about 5fps difference at initial release. Feathers will be faster. Vanes start out slower but because they create less drag theywill retain more velocity over longer distances. So vanes will be moving the same speed as the feathers, generally at or about 50-60 yards. The feathrs will create more drag stabilizing the arrow better making for better accuracy and grouping, especially with broadheads. Feathers, being lighter in mass weight will still work well when they are beat up, chewed up, and sometimes missing (as if one comes off the shaft leaving only two) because they don't throw the balance off so much. Not so with vanes. Even with a piece torn out the balance can get real critical.
A few years back I had a bow really, really tuned and could shoot arrows with only one or two feathers, and yes, even bare shafts at a 3D shoot. Out to 35 yards it just didn't matter. Not many bows tune that well, but I guess I got lucky on the setup. But that's just another reason I shoot feathers.