Since I missed Old Stuff Day, here's some belated Old Stuff - a Vyper Review from several years ago...

Before we begin... I haven't read this through thoroughly, it's a copy/paste job of an old piece of work destined for my Complete Guide to the Eldar.  There exists, therefore, the possibility that it makes the Vyper out to be better than it is, or that my opinion has changed somewhat regarding the utility of certain options.  Obviously - it's worse now than it even was then.

  Vyper Squadron:

Basic = Strangely, Vypers come with an Underslug weapon, and a mandatory decision for a turret, with no free base weapon.  This is not in any way unusual in the context of the Eldar Codex, all Eldar vehicles are the same, except the Fire Prism.  It is unusual, however, in comparison to other Codices (Razorbacks and Land Speeders in C:SM, for example) and indeed in comparison to the previous Edition of the Eldar Codex, in which a Vyper tooled with the bare minimum of Catapults underneath (who ever thought they were an appropriate vehicle mounted weapon? Seriously…) and Shuriken Cannon on top, it actually comes out more expensive now than before with no additional benefit.  Being subject to Vehicle Squadron rules when you’re already Open-Topped = bad vehicle design, btw.  Fire a long range Multi-Melta, and hit me.  Then, on a 3+ you pen…destroying me on a 2+ with the subsequent roll.  Even a non-AP1 weapon has a better than 50% of destroying on a Pen.  Maybe if something about as survivable as an Ork Trukk didn’t cost at least as many points as a Land Speeder (without BS4, being Open-Topped, and without the ability to Deep Strike…)

Turret weaponry : 

Shuriken Cannon = the cheapest option, and probably the most versatile.  Take one of these on top, one Underslug, cheap as really expensive chips, and capable of laying down a withering hail of fire that threatens Rhinos and GEQs alike.  Even MEQs would die to the hail of shuriken, if you were left alone long enough.  

Scatter Laser = (Yeah, I know I’m doing these out of the order they appear in the book, but they’re in a stupid order.) The next most viable option.  The additional range and RoF over the Shuriken Cannon makes for this variant to be slightly more survivable, hiding, as it will, approximately 35.5” from the foe.  If only the Vyper wasn’t shaped in a way that made it hard to fire this at targets in front of you without putting them in range as well.  Of course, your armour is equally weak all round, so no huge loss flying sideways.  I’m unconvinced by the Underslug Cannon on this build - if you lose the Laser, flying in closer may be your only option however…probably a good idea.  

EML = Ah, the ubiquitous EML.  With it’s 48” range, and variety of firing modes, this weapon seems perfectly suited to a long range sniping platform, equivalent in some ways to the Space Marine Typhoon.  It’s significantly inferior, of course, but it’s also a damn sight cheaper, so it’s almost worth using.  The problem is that you should have managed to get together enough anti-tank from the Wave Serpents, and, as an a-I weapon, the EML is inferior to the previous two options.  

Bright Lance = If it weren’t outranged by the EML, the Lance would be a pretty decent choice on the Vyper despite the ridiculous cost incurred.  If you desperately need more ranged anti-tank, not a terrible selection…assuming you’ve maxed out of proper BL Platforms.  Since you can take a maximum of 14 Wave Serpents in an army, I think you should skip to Part Seven, and the Apocalypse section.  

Starcannon = Oh yes.  I left it last on purpose.  This is the single worst choice, it’s ridiculous.  Now, I know Fritz at WOSH loves it…but he admits it’s less effective than a Scatter Laser in it’s place unless you get fortunate enough to be shooting MEQs out of cover.  This weapon has no place in ANY competitive Eldar list, least of all on a BS3 Platform it makes 70 points.  Open up Codex: Space Marines there, turn to page 139, and look at the variety of Land Speeders you can have for 70 points - with TWO weapons, not one and a retarded 12” one.  Oh look, a MM/HF one, the most popular, and IMO, best.  Only the Piranha is a rival to that Skimmer for the role it performs, and the Vyper is stuck trying to snipe a maximum of 2 guys a turn from 36” away, hoping you don’t get Cover saves.  There is NEVER a justification for a Starcannon on Vyper in a Tournament list.
Underslug Shuriken Cannon = Yeah, this is good.  Overcosted, but good.  Take it for any Vyper that isn’t just going to hug the board edge, hoping to get lucky with a S8 shot at Tanks.
Vectored Engines = This is a gun platform.  Literally, just a glorified HWP on a Jetbike chassis.  You’re destroyed on a 4+, at BEST, because you’re open-topped.  Don’t make it cost more than it needs to.  In fairness, this principle applies to every upgrade here, you don’t need any of them.  

Star Engines = again, if you aren’t shooting with it, it’s because it’s dead.  

Holo-Fields = Hahaha.  80 points, + weapons.  Nu-uh.  Doesn’t even really increase survivability on a Vyper, either.  

Spirit Stones = Being Stunned is the least of your worries.