I have played a ton of games with Vayl2 and I love her, Ed has not so much love.
Spells and Abilities
Vayl (much like my other love Caine2) is a squishy low ARM caster who loves to play attrition but will assassinate the shit out of you if you give her half an opening (and she really only does need half an opening). Much like Caine you want to save her feat for late game when you can assassinate a warcaster unless you need it to remove a key model from the table. Unfortunately her feat does not have quite the hitting power of Caine's feat but if you combine two boosted Ravagore shots with two boosted Obliterations you can take down some heavy targets.
Her main spell as far as game-play is concerned is refuge. That spell is huge in Legion. You cast it on an Aneglius which is SPD 7 and has reach and it can be 9" away from the model it attacked. Casts its animus before it refuges and it pushes everything 3" further away before it moves so it is 12" away from enemy models. She can cycle that spell between two Angels and they become very potent and very hard to remove. She also has Occultation and Admonition (stealth and a spell that allows you to move away when a model comes near your model). The combination of those spells makes her an excellent denial caster. Icy grip helps tremendously with the lowish MAT of Legion beasts (-2 DEF, offensive spell and stops special attacks like bile thrall purges) and as part of the assassination run and Purification is still a ball busting spell of uber ridiclousness.
Army Composition
You want to go for the theme force here because it is almost exactly what you would be taking anyway. You want double Angels for the repulse/refuge trick, Admonition is another spell that supports heavies and the spawning vessel is fantastic with an assassination minded caster. Other than the double Angelius you want something to feed the spawning vessel, something else with flight for the theme force bonus, then fill with points discounted heavies.
At 50 points I recommend strongly to go double Ravagore. I spent a lot of time with Vayl playing two Angels and a Seraph as my three flying beasts because that made a lot more sense to me (points discount, threat range extender and movement shenanigans for the assassination) but she the points are better spent on ravagores. The Ravagores do a lot of damage from range, the Angels retreat again giving you distance, they complement the list quite nicely and their ability to shoot at casters complements the assassination more than the movement shenanigans of the Seraph. Vayl already has 25" on the assassination, she doesn't generally fall short. The issue with the Seraph is it doesn't do enough. It slipstreams and then shoots at infantry, poorly. A bit extra range would help, a damage buff so it could shoot at heavies would help. I don't know how much SR 2013 changes this for her, it changed my 35 point list, putting in a Scythean rather than a Ravagore for more forwards presence (and now I am trying an afflictor instead of the Sorceress and Hellion as well) but I haven't played enough of her at 50 points in SR2013 to be 100% sure. She doesn't struggle too much with scenario because the double Angelius makes it easy to clear a zone, she can spawn a lesser and launch it into the enemy zone without losing anything and walking into her threat ranges is always painful. The afflictor creating solos and then refuging away is great for scenario.
If you do take her out of her theme force take Raptors. They are silly with Occultation. That's the only weakness of her theme force, the lack of raptors.
Things to Avoid
Carniveans. I don't know why this beast gets so much love on the legion forums. It's like a scythean except worse and costs more points. You don't need spiny growth, take the reach beast.
Throne of Everblight. It's in her theme force and some people are tempted to use it to screen her. Don't. It's pretty good if you can armour stack it but she can't.
Light warbeasts other than the afflictor. You get a points deduction for heavies and legion heavies are so much better for their points than our lights. The afflictor gets a pass because you need a third flying beast for the theme force and it helps with the low model count. The other option is the Harrier, both are fine.
Hex Hunters - Stealth actually becomes a problem when you have a squishy caster with a low model count. She has little synergy with them and often wants to kill her own models to create lesser warbeasts, the cheaper Legionnaires are better for that as well.
Tips for Playing Her
She doesn't piece trade, she removes pieces without risking her own. In that vein don't risk your pieces. Refuge your angels, use your ranged firepower, use admonition, force your opponent to come to you and tear them apart piece by piece.
Try to hold onto the assassination. The Angels can shoot, as can the Ravagores, keep them alive, remember the massive range of the spawning vessel, it contributes too and you can kill your own models to spawn that lesser warbeast. If you use the feat you lose a lot of assassination potential, avoid using it unless you really need it.
Look for scenario, if your opponent doesn't have an out for stealth get her into the zone. It helps threaten the assassination and lets you dominate. Use the repulsion on the Angelius and remember that you can charge with vayl, hit something, repulse, refuge and then repulse again. Vayl herself can move a model 6" which will get it out of most zones and away from any flags or objectives.
The biggest mistakes I have made with her have been trying to do too much. She has a very good long game, use it. If you bite off more than you can chew you lose more than you are prepared to. Be happy to pull your opponent apart slowly as long as it keeps your force alive.
Other Playstyles
The only one that really counts in the Seraph. I don't like it at 35 in 2013 and I didn't like it at 50 in 2012. I don't think the scenario changes make a difference, Vayl is still a scenario monster.
My list
Legion of Everblight
Machinations of Shadow (Vayl, Consul of Everblight)
(Tier 4)
PCs: 50 / 50
27 models
[00] Vayl, Consul of Everblight (0 / 6 WB)
[09] >> Ravagore
[09] >> Ravagore
[08] >> Scythean
[08] >> Angelius
[08] >> Angelius
[02] >> Harrier
[01] Blighted Nyss Shepherd
[01] Blighted Nyss Shepherd
[01] Spell Martyr (1 Martyr)
[06] Blighted Nyss Legionnaires (9 grunts)
[03] Spawning Vessel (6 grunts)
Pretty much all 50 point Vayl2 lists will be this exact list. It's just the way the points work out with the theme force bonuses and requirements. I really want to put an afflictor in for the Harrier but though the spell martyr can go (it's there as the only one point option) I don't have another point in the list.
Match Ups and Pairings
I have had huge difficulties finding a pair for Vayl2. I think Thagrosh1 would probably be my go to pairing but meh. In a three list format it's easy because I can take Vayl2, Lylyth2 and any of the warlocks that like to get forwards (Vayl1, Saeryn or Thagrosh1).
Her main two weaknesses are colossals (the will always be able to get to your angels if you refuge away from them) and armies that have an out for stealth and can play for scenario. As such she hates playing against Legion and Cygnar in particular. If your opponent has no guns she will walk all over them. There is no easier match that Vayl2 against a list without guns.
