CVinton over at Dark Future games did a nice article on using little used units, such as the Vyper. One of the nice things about the Eldar codex is that even the worst units in the codex usually have some value. They tend to be overpriced but are usually capable of doing something if used properly.
The Vyper is probably the exception to this rule. Open-topped and Armor 10 make it to fragile to get near the enemy, BS3 makes it too unreliable to shoot the enemy. About the only good thing you can say about this is that is such a non-threat that you don't have to worry about it getting blown up so it can contest an objective late game.
My favored setup is to give it a cannon on top and bottom. This is a relatively cheap layout and should net you 3 S6 hits per turn, which translates into 1 glance/pen on a rhino. The other use I've found is to fly it up directly in front of the opposing transport to force them to try and ram you if they want to advance. You get a 3+ dodge which, if successful, stops them from moving and when the Vyper dies it creates difficult terrain to hopefully immobilize the vehicle. I try to avoid this as it does make sense to me in a narrative how missing a vehicle you are trying to ram stops you but at times it is definitely useful.
Anyways for league I armed mine with a missile launcher. This is probably the second best layout IMO as the range of the missile allows you to stay way back until it is objective time and if you are fighting a horde or something that can be pinned the plasma missile option minimizes the crappy BS3. In my first three games it has lived up to expectations. AKA not doing much other than getting extremely lucky and contesting an objective in one game.