TACTICAL SUGGESTIONS:

When using Vypers it is best to use more than one at a time, as they are fragile, and can be shot down with ease. Using cover can be a way to make Vypers last longer. Vypers are particularly vulnerable to blast marker weapons as they are considered to be "open topped". If multiple Vypers are used in squadrons, more than one at a time can be taken out by one blast marker weapon or enemy squad.

One way to avoid heavy losses to incoming fire, is to put only one Vyper in each squadron. An enemy unit firing at the Vypers must shoot at one unit at a time, and therefore you will only lose one Vyper at a time instead of up to three. Of course, keeping the Vypers moving over 6" will turn all penetrating hits into glancing blows.

Vypers can be considered as either a fast trouble-shooters that can respond to threats quickly and effectively or as a surgical strike force that can remove troublesome enemy units.

Another use for the Vypers is as a harassing force.  Usually when an enemy sees a few squadrons of Vypers with heavy weapons they tend to assign the Vypers as a high priority threat and therefore go to a fair bit of effort to destroy them.  When the enemy spends a lot of fire power shooting at the Vypers, the rest of your army can move forward with less risk.  This tactic is especially useful against Imperial Guard.   No Imperial guard player will feel comfortable with four or five Vypers in two or three squadrons, armed with bright lances heading towards their tanks.  As the Guard player shoots at the vypers, you can move up your hand to hand troops in vehicles up to 24" (if you forego shooting) closer!

Although Vypers are fragile and they loose their main weapon easily, they can still be useful for cross fire purposes with only the shuriken catapults.

WEAPONS CHOICE:

The type of enemy will determine which weapons to arm the Vyper with. If you are using more than one Vyper in a squad, then arm them all, with the same weapon. The targeting rules state that a squad must fire at the same unit. Just imagine that you've just shot at a Leman Russ with the lascannon, and missed, and the other vyper in your squadron has a scatter laser. The scatter laser will be useless against such a target, and therefore a waste of points.

As with most of the Eldar army, it is better to specialise and make the Vypers task specific.

Since it is wise to keep the Vyper moving more than 6", it is a waste of points to up grade the Vyper 's shuriken catapult to a shuriken cannon, as only one weapon can be fired.

If you're playing against a heavily armoured foe such as Imperial guard, a Bright Lance would be a good choice. As tanks will be your target. The Bright Lance is a better choice in most cases than the missile launcher because for 5 points less you get a better chance to penetrate tanks and terminator armour.

Against all other armies, a Star cannon would be a versatile choice. Star cannons are very good against heavily armoured troops like Marines and light vehicles. Star cannons are the antidote for terminators.

As for the Eldar missile launcher, I think that even though it has greater range than the bright lance, the Bright lance has a better chance to penetrate thicker armour and it is cheaper in points. Against power and terminator armour the Star cannon is better than the missile launcher.  The Eldar missile launcher is a good choice against armies like Orks or Dark Eldar that have lightly armoured troops that can be pinned by the plasma missiles.  Orks and Dark Eldar also have lightly armoured vehicles most the time that don't need the extra armour penetration of the Bright Lance.

A squad of three Vypers with shuriken cannons would be cheap option against armies with massed troops with low armour saves, such as Orks, Tyrranids and Imperial Guard.  The trouble would be getting close as the range of Shuriken Cannons is only 24".