This is somewhat King Elessar's fault, mostly for making me think more about Vypers. I've also run them some lately, and been reasonably pleased with the results. That in mind, I think I'll break it down and do another evaluation of the unit, and/or Squadron of Vypers (because you can indeed slap 3 into the same force org slot.)
The Vyper Chassis
Naked, a Vyper is 45 points. It's a small AV10 all-around open-topped fast skimmer. Its gunner is apparently an Eldar civvy; it's got the almight BS3. It packs a chin-mounted twin-linked Shuriken Catapult, though I cannot for the life of me tell you why it does this. Perhaps it's to give the pilot some piece of mind, but he's at least got that canopy. The gunner's stuck with his butt in the wind (...somewhat literally, as you can see) and it's bound to be both thrilling and an incentive to buy plenty of underwear.
It's cheap, it's Fast, it's a skimmer. It can move 12" a turn and fire the main gun (...and if you're suicidally close, the secondary gun). It can move 6" and fire two main weapons, and we'll get to the part where it has two later on. It can also bust out a 24" move if you really need to reposition.
If you didn't notice already, it's AV10 open-topped. Bolters and other small arms are a legitimate threat. In short, Vypers die when the other guy gets a shot at them and decides he wants them to die. Hence the 'boom' part at the end of the title.
Vyper Weapon Options
Vypers have a turret mount, and you must slot something into the turret. You may also drop 10 points to upgrade the chin-mounted twin-linked shuriken catapult to a shuriken cannon (much like any other Eldar skimmer...). For the turret, then, we can use a shuriken cannon, a scatter laser, a star cannon, an Eldar missile launcher, or the good ol' Bright Lance.
I favor the scatter laser or shuriken cannon approach. With BS3, you want volume of shots so some stick. The turret-mounted shuriken cannon is a mere 5 points, and if you want another 12" of range and fourth shot, you get the Scatter Laser. The Star Cannon just doesn't do it for me, since there's this little thing called Cover.
The EML is kind of in the middle; it's so-so against armor and so-so against infantry since it has the S4 AP4 Pinning plasma shot.
The Bright Lance is frightening, but really wants Guide, and makes the base Vyper 75 points.
Kitting Out your Flying Death Trap
So, we know that the Vyper is swift, fragile, and carries a big gun (or two). The next question is, what do we do to it?
Theoretically, it can take the gamut of Eldar skimmer upgrades, like Star Engines, Holo-Fields, Vectored Engines, and Spirit Stones. Theoretically, you could throw all those on and make your Death Trap cost a lot more, but...why would you? Hell, Holo-Fields cost 35 on this thing! Bottom of the line for non-gun options?
None of them hange the fact that you are AV10 open-topped, and go away when someone shoots at you enough.
Now that we've got that out of the way, let's look at some configurations...
Double Shuriken Cannon (60 points)
You upgrade the chin gun to a shuriken cannon, and take one on the turret. It drops six S6 shots at 24", and has the speed to get there. You might spend the first turn getting into position, but past that you should be hosing down the enemy infantry or light transports with the guns. It provides you with a good bit of medium-strength gunfire for this, when your other guns are doing other things. Amazingly, it can also sustain a weapon-destroyed result and still be a threat.
Scatter Laser (60 points)
Same cost as the double-cannon model. I resist putting a cannon on the chin to minimize the cost, and to resist the urge to get to 24". Here, you stay further away from the enemy. Same basic mission as the double-cannon model, just longer range and fewer shots. I think these two are pretty interchangeable.
EML/Cannon (75 points)
This one is theoretically more versatile, as it can move 6" and pop off three S6 shots and a single S8 shot, or 12" for three S6 shots and the plasma missile. It can sustain the loss of a weapon, and remain a threat of sorts. If we're hunting armor, though, the EML is nasty vs. AV10/12, but not significanctly nastier vs. AV 10/11.
Why is that?
BS3 Missile Launcher vs. BS3 Shuriken Cannon
vs. AV10: ML pens 33% of the time; cannon gets a single pen 75% of the time
vs. AV11: ML pens 25% of the time, cannon pens once 25% of the time
AV12 is a glance-fest for the cannon, and the single missile pens it 16% of the time. The missile launcher can at least pen AV13, and might even damage AV14, but why bother? A Lance beats it out any day out of the week on the higher values, does the same at lower, and trades out the silly plasma missile.
Bright Lance (75 points)
We all know how fun bright lances are. So, why not field another one? Well, there are a few things I can see going wrong here. First, it's BS3. Not 100% reliable. Second, it's a bright lance on an AV10 open-topped platform.
I think if you're taking a Bright Lance on your vyper, you don't much love your vypers, because they are fragile and the Bright Lance has a slight psychological impact.
The Flying Death Trap in your army
It goes without (much) saying that any gun in the Eldar army that isn't twin-linked could always benefit from a good bout of Guide from your friendly neighborhood Farseer. I think Guide is a pretty solid Farseer power anyway, so it has a place in your army. Pretty much anything other than Wave Serpents really benefit from it (since their Lance already has a re-roll). I consider Guide/Doom the Farseer powers for a mech army, and naturally Fortune/Doom go on the Jetbike Seer Council Seer, and he's a bit exclusive from the Vypers.
If, for some reason, you feel like turbo-boosting them, you could always benefit from Fortune on here, but that mostly just means someone will shoot something else. Or, if the Eldar player is using them to contest, the enemy STILL might pump enough shots in to do the trick, since it's still AV10 open-topped and small arms are fair game. (And, of course, we're ignoring the fact that the 'Last Turn' is not always the last turn, unless you know it is because of time constraints, or turn 7, etc.)
We know that the Farseer is your buddy, but that's just an Eldar truism. Vypers are mobile, and compliment a mechanized army.
They also compliment a mechanized army because as I've said, Vypers die pretty easily. There needs to be that natural saturation of armored targets. They are a compliment; they can be used as a screening unit in deployment, they can help crack transports, or they can get at the juicy bits in freshly-cracked transports.
In this capacity, I use them with the double-cannon or scatter laser model. 24" is probably enough; just commit to moving 'em into position. Additionally, if you're going full mech, you probably have 3-5+ Wave Serpents, all with lances. Shuriken-cannon Vypers give you some much-needed volume of fire, here.
I suppose in larger fights (IE: Apocalypse) where the Force Org chart isn't an issue as much, and the points are, the Bright Lance models are worth a bit more. More firepower if you want it. I don't have a lot of experience in Apocalypse, but it seems like the psychological impact of the Bright Lance and the fragility of the Vyper might be handy here.
Vypers in a Squad
Now, you can theoretically take up to three Vypers in a squad. It goes without saying that they should be equipped alike; they're shooting at the same thing, after all.
The squadron rules, as they are, essentially grant you extra armor; you can always move. However, you also lose the vehicle on an 'immobilized' roll. What does this mean to you?
Don't take Vyper squads unless you're attempting some kind of 'torrent of fire' army* and you really want a lot of Vypers. AV10 open-topped is just not squadron material in any normal sense. Why? Simple. The way weapons work. Weapons from S4-7 tend to be higher rate of fire, and often come in units. The perk of a squad is that you get to distribute the hits.
Spreading out the love is handy in, say, a two-tank Leman Russ squad. Most units other than Fire Dragons put one penetrating hit out at a time, and you can slap them all on one tank. IE: my buddy Jon brought a Pask-terminator (the one with the heavy 3 turret plasma cannon, plasma sponsons, and Pask for the re-rolls and such...) with a regular Russ. Guess where the damage went? (It was only worse for me when I'd brought Wraithguard......but that's a digression for another time.)
But, for Vypers? A single decent autocannon volley is fully capable of blowing away two Vypers. Take something like the Dakka Pred (Autocannon, 2 heavy bolters) and it can SHRED Vypers. +1 to the damage results and death on immobility means that you only need a 3+ to kill a Vyper in a squad if you penetrate it.
Bottom line? Don't take Vypers in squads unless you really want to maximize their numbers.
Summary on Vypers
Vypers are nasty, fast little boggarts. They benefit from maximizing the rate of fire, rather than potency as they are, of course, BS3. They are a fine compliment to a Serpent-heavy mechanized army, adding volume of fire to the quality of several twin-linked BS3 lances.
Or, the more succinct summary? A Vyper's life cycle in three parts is:
*I'm talking like massed war walkers/vypers for a crap-ton of S6 dakka, and then probably some fire dragons/avengers in serpents for scoring/tank busting. I do not have experience playing this type of list, but think it's got promise if you really want to invest in a ton of walkers/Vypers. (...though, if you go double-cannons on 9 vypers and double-scatter-lasers on 9 war walkers, you are looking at...well...126 S6 shots in a turn. Sure, it blows up easy, but if you bring the means to kill AV13/14, it's kind of nasty).