Vanille – Synergist Crystarium

Character Vanille
Role Synergist
Along with Fang, Vanille is one of two Characters that has access to Bravera, Faithra, Protectra, and Shellra, high-powered Abilities that have a short duration to balance their extra effectiveness. Unlike Fang, Vanille has a decent number of standard Synergist Abilities as well, including Veil, Vigilance, and the Bar-spells. She is the only Character in the game that doesn't get Haste, however, so she's not very effective as your only Synergist.
Compare Vanille – Synergist Crystarium to other Crystariums

Related

Vanille

Eidolon Hecatoncheir (Chapter 11)
Base HP 350
Maximum HP 21,050
Base Strength 23
Maximum Strength 1,400
Base Magic 35
Maximum Magic 1,700
Starting Weapon Binding Rod
Primary Roles Ravager, Saboteur, Medic
Accessory Crystal Saboteur 5
Accessory Crystal Medic 8
Accessory Crystal Ravager 10
ATB Level Crystal Medic 8
Chapter 1

Fang – Synergist Crystarium

Character Fang
Role Synergist
Fang is an interesting Synergist that can be quite effective despite having less Abilities than any other Character. She has access to Veil and Haste, but more importantly she gets the high-powered Abilities Bravera, Faithra, Protectra, and Shellra. These Abilities, exclusive to Fang and Vanille, are more powerful versions of the Abilities they are based on, but have a much shorter duration. You'll have to pay a lot more attention to your buffs if you use them, but the payoff can certainly be worth it.

Hope – Synergist Crystarium

Character Hope
Role Synergist
Hope, like Sazh, eventually gets almost every basic Synergist Ability (though he gains Veil over Sazh's Vigilance). Until Chapter 10, however, Hope is primarily a defensive Synergist, with Abilities like Protect, Shell, and Bar-spells. Which of the two is more useful depends largely on how hard the enemy in question hits.

Lightning – Synergist Crystarium

Character Lightning
Role Synergist
Lightning is a surprisingly effective Synergist, gaining all of the basic Abilities with the exception of Bar-spells. Her biggest failing is not learning Haste or Vigilance until Crystarium stage 10, however. Once all is said and done, she's quite effective simply because Bar-spells aren't of much use against the toughest enemies anyway. Too bad there's no Barstone in this game.

Sazh – Synergist Crystarium

Character Sazh
Role Synergist
It takes him a while to get there, but Sazh is the most complete Synergist in the game. He and Hope have very nearly the same set of Abilities, though Sazh gets Vigilance instead of Veil. However, Sazh will first gain access to offensive buffs like Bravery, Faith, and En-spells, making him the offensive Synergist for the first nine chapters of the game.

Snow – Synergist Crystarium

Character Snow
Role Synergist
Snow is a decent Synergist as long as you don't care about elemental buffs—he doesn't get any Bar-spells or En-spells, nor does he get Veil. He does have the standard Protect/Shell and Bravery/Faith pairs, though, so he's a useful enough Synergist in a pinch.

Vanille – Commando Crystarium

Character Vanille
Role Commando
Vanille is the most limited Commando in the game (and has the silliest Weapon), lacking both Launch and Blitz. She does get Blindside and Faultsiphon, auto Abilities that are easy to take advantage of. However, her Role should generally be limited to that of a secondary Commando. She's more useful for the Augmented Attack effect than actual Commando Abilities.

Vanille – Medic Crystarium

Character Vanille
Role Medic
Vanille and Hope are the only Characters with access to every Medic Ability, and while Hope's magic stat is naturally higher, Vanille is plenty effective as a Medic as well. She and Hope are the only Characters who can use the amazing Curaja Ability, a vital healing spell in the endgame.

Vanille – Ravager Crystarium

Character Vanille
Role Ravager
Vanille is a good Ravager with a number of holes in her Ability list. Like Hope, she lacks any -strike Abilities, making her unable to damage enemies that are immune to Magic. She can still advance their chain bonus (and doesn't need to worry about strength!), but this can be a big problem against certain foes. She also lacks Aeroga, Thundaga, and Waterga, and doesn't get the Vigor Ability. None of this is crippling, but it does set Vanille firmly behind Hope as far as Ravagers go.

Vanille – Saboteur Crystarium

Character Vanille
Role Saboteur
Vanille is a very effective Saboteur with a variety of Abilities, though she is more concentrated on defense-lowering debuffs. She cannot inflict Slow, Curse, or Daze, and also lacks Fogga and Painga, but has single- and multi-target versions of every other Saboteur Ability. Beyond that, she has Death, a unique Ability that either just kills a target or deals a large amount of damage to them. Since very few non-boss enemies are actually immune to Death, you can get cheap, fast kills against a number of difficult foes (up to and including Adamantoises) with it.

Vanille – Sentinel Crystarium

Character Vanille
Role Sentinel
Vanille as a Sentinel? Surely not! And truth be told, she's not very good at it. She has Mediguard at least, but not much else, including Challenge. She does have more HP than Lightning or Hope, but both of them are evasion Sentinels anyway. Basically, you don't want Vanille to be your main Sentinel. She's fine in the Tortoise (SEN/SEN/SEN) paradigm, but not much else.

Synergist

Bonus Type Extended Enhancements

Synergist

Abbreviation SYN
Role Bonus Extended Enhancements
Base Bonus (Self) None
Role Level I Bonus (Self) Duration Bonus: +15%
Role Level II Bonus (Self) Duration Bonus: +30%
Base Bonus (Allies) Duration Bonus: +5%
Role Level II Bonus (Allies) Duration Bonus: +8%
Role Accessory Supporter's Amethyst (Grants Duration Bonus: +20% to self)