How important is the length of vanes? Would longer vanes be better than short for broadheads? What is the difference betwen straight, right, and left helical? Could someone tell me why you would use one over the other?

There are many considerations here on vanes. It all depends on your present setup. Arrow weight to lbs your shooting for proper penetration. Forward of Center percentage on your arrow (FOC), and rest clearance.
Helical vanes or offset helps to spin the arrow for better accuracy. Somewhat like a bullet leaving a rifle barrel to speak. Straight vanes won' t give you the spin as fast, and could spin in either direction once leaving the rest. But straight gives you better rest clearances. So how much degree of offset on your vanes depends on your clearance. Latest tests have revealed left or right helical is more of a personal perference, and neither makes much of a difference. The old thinking was it depended on which hand you shoot. I hope his helps.

First off, nice to see another Michigander here! Length of veins is dictated by many things. For example...I shoot a 410 grain arrow 280fps using a 3 inch straight feather.
Target tips are no worry...I don' t have a big broadhead trying to steer my arrow. But when I put on a 2" cut on contact broadhead those 3 inch straight feathers aren' t going to cut it.
Come hunting season, I' ll put a 4 or 5" hellical on and the added drag and stability will straighten those big heads right up.

Those big, hard hellical fletches are notoriously hard to get clear of the rest unless you have a drop away though.

Peterson' s Bowhunting ,march 2003. went int detail om this abd on the feathers.
here is the web site it may be there also www.bowhuntingmag.com
I thought this was a very informative artical.
thanks

I' d also recommend reading the article " Flight Fletching" ,
by Dave Dolbee, in the new Petersen' s Bowhunting-
p. 130 (March/2003 issue). Very informative as various
tests are done using the " Hooter Shooter" .

as others stated interesting article. I honestly like a good vane like a 4" duravane or AAE for most hunting. I use a hard offset since I like launcher style rests and helical is a bit too much to get between the prongs. Nothing honestly flies like a feather but you gain some things & looose a few things too. try both & play around a bit to see what you like!