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.