Well, longer vanes on a helical give the most stabilization because they disrupt the most air. However, there comes a point when you have all that is needed to stabilize your setup. For instance, if you are target shooting, you will probably only need a 2 or 3" vane to correct your arrows. Now, if you are shooting big fixed blade broadheads, you are going to need more.
Helical adds spin to the arrow. Here again, causing more air disruption and stabilizing the arrow. There is a negative to both. That same air disruption that stabilizes the arrow also slows the arrow down. So there comes a point with helical that too much is not doing any good either.
Feathers are even better yet, and they weigh much less than vanes. So at distances up to about 40 yards, you will gain a little speed because of the weight difference. After about 60 yards, your arrow will lose speed more quickly than vanes because of the air disruption that feathers create. (wind resistance).