RE: vanes
Feathers have the advantage of forgiveness to contact. On the other hand they have a problem with moisture. They are also not as durable.They tend to get ragged from use and even being in a quiver.
2" vs. 4" - I talked to a guy at a pro shop once that told me about Blazers vs. 4" vanes. As the story goes, the wind was blowing at a 3-D shoot. On one particular target there was protection from the wind all the way to the target except for a short distance where the wind had a clear shot at the arrow in flight far a little way. Apparently there was a noticable difference between the Blazers and 4' vanes in the wind (imagine that!).
All that said, as previously mentioned, they all work. I have had 2" vanes, 4" vanes, and 4" feathers all in my quiver at the same time (a flecthing study). Granted I'm not the best shot in the world, but I couldn't shoot the difference in any of them with 200 grain Muzzy Phantom SS broadheads out to 60 yards.