| Bonus: Void Gear Keyblade
You can do this the easy (and cheap) way or hard way. Should you
choose the former all you need is a healing spell and bunch of
Strike Raids. For the latter, prepare the following.
* I would recommend training your character of choice to level 40.
* For abilities get Second Chance, Once More, Item Boost, Finish Boost, multiple copies of Magic Haste (for Ven and Aqua).
* Get best weapon and Finish command if possible.
* Replace elemental action commands with mastered basic ones, as the their elemental counterparts don't grant temporary invincibility.
Vanita's attacks are as follows:
01. Vanitas takes cover within a darkness orb and produces seven
illusions of himself. These illusions will then charge at you
and inflict blind if they connect. As soon as all 7 are done
doing their thing, Vanitas himself will rush at you for a combo
Dodge the Vanitas copies by rolling, cartwheeling, or sliding
out of the way as soon as they charge at you. Then repeat when
Vanitas takes his turn. Or just mash  all throughout. Vanitas
is also invincible while inside the orb.
You can interrupt this attack altogether by smacking Vanitas
before he gets a chance to initiate the move. Try going for a
more aggressive approach at the start of the fight in an effort
to cut this attack off.
02. A two-hit combo followed by a sonic blast. You can block the
first swipe and then use Counter Rush. All succeeding keyblade
swipes are unblockable though. The sonic blast will also
inflict confuse if it connects.
03. Vanitas holds his weapon over his head and charges it for a
complicated magic-attack combo. First he charges you while
protected by a large circle of flames, then he does a second charge,
releasing a homing blast of fire your way. The best approach for this is to wait
out the first half of the attack from a safe distance, then move in during the
homing blast part. Combo Vanitas when he lands.
04. He fires one large fireball which splits into 5 high speed
homing ones. Dodge Roll away the moment it splits to avoid. You
can also block the big one and keep it from splitting. The move
can be interrupted by hitting Vanitas before he releases the
05. Jumps into the air and fires a shower of dark lightning. This
only hits you if you happen to be a few feet away from him. Get
closer and the attack misses completely.
06. Dark Spiral -- He surrounds himself with a blue aura, and
performs three consecutive aerial charge. Dodge roll to avoid
then wait for him to come back down and nail him with a combo.
07. Vanitas teleports nearby and moves around you at high speed,
producing illusions of himself that will blind you should they
find their mark. Regular dodge actions will save you from this.
08. Dark Splicer -- Vanitas teleports behind you for a combo. Block
this and fight back with Counter Rush. If the target lock gets
disengaged and you lose track of Vanitas, keep moving with
regular dodge moves. It'll keep you out of trouble well enough.
09. Vanitas does his own shotlock which fires a short-ranged laser
that slowly circles around. Take this chance to nail him with
your best attacks, as he will not pull off a sudden counter
move for the duration of this attack.
10. Vanitas burrows himself into the ground, tailing you at high
speed then emerges with an upward slash before following
through with Firaga Burst. Move over him with regular dodge
moves so that you will be temporarily invincible when Vanitas
pops out, then quickly counter with your own string of combos.
11. Vanitas still has his classic dive bomb attack, but this time
it is ice-elemental. He does this after leaving an illusion of
himself, usually after your combos hit him a few times. Dodge
the moment he teleports to avoid the upcoming counter move. He
might also rush you with another melee combo right away, so
keep mashing .
12. Vanitas has the ability to emulate you when you heal yourself
with magic and items, but only when he sees you do it. So go
hide behind one of the boulders to stay out of sight then heal.
Bringing a ton of healing items is a better choice than magic.
Set mines around the arena with Mine Square/Mine Shield, and bait
Vanitas so that he steps on your traps. Though this does little
damage, it is an effective way to fill your command gauge until
you can nail him with your main weapon, the Finish command.
For Ventus, you will also need Mine Square/Mine Shield and Tornado.
Vanitas is one slippery fellow, so nailing him with Tornado can be
somewhat of a challenge. For this you have 3 options:
* Bait him into stepping on your mine field so that he goes flying. Nail him with Tornado while he's airborne.
* You get a pretty good opening at the end of attack #03; run up to him as soon as he lands and nail him with Tornado.
* Run up to Vanitas during attack #09 and cast Tornado.
In Aqua's case victory will depend entirely on Seeker Mine. As
always, lure him into your traps, but there are 2 instances where
Seeker Mine is most effective.
* When Vanitas does attack #09, take a leap way over him and let the mines loose. Vanitas will take the full brunt of the attack.
* During attack #10, drop Seeker Mines while Vanitas is underground, then wait for him to arrive, dodging at the absolute last second so
that Vanitas pops right out on your traps. He will also take the full brunt of the attack here.
Exploit Trick for the Cheap
For this one all you need to do is fill your deck with Strike Raids
then find a boulder and hide behind it. From behind the boulder,
spam Vanitas with Strike Raid until he dies.