The Mirage Vest is a top tier light armor in Final Fantasy XII that boosts the wearer's defense by 55 (Zodiac) or 45 (original), HP by 800, and Vitality and Speed by 10. It can be considered the best light armor in the game as it has the highest stats, though doesn't offer any elemental protection like the Rubber Suit, or any buffs like the Brave Suit; it is up to the player's consideration which light armor to use in the endgame, if they are using light armor over heavy and mystic gear.
The Vitality and Speed boosts could be useful for those who wield weapons that take these stats into damage calculation: axes and hammers and hand-bombs for Vitality, and bows and daggers and ninja swords for Speed. However, a heavy armor that boosts Strength has a bigger impact on the damage dealt for all weapon types, and in the Zodiac versions, the Foebreaker—who uses axes, hammers and hand-bombs—can't natively equip light armor.
In the original PlayStation 2 versions, the Mirage Vest is sold for 16,000 gil in Lowtown after obtaining the Treaty-Blade. It is found as a treasure in the Lhusu Mines' Site 5 (always appears, 10% chance the item treasure is the Mirage Vest with the Diamond Armlet equipped ), Cerobi Steppe's Old Elanise Road (50% chance to appear, 50% chance for gil, 10% chance the item treasure is the Mirage Vest with the Diamond Armlet equipped ), and Great Crystal's Sirhru Jilaam Praa'vaa (always appears, 70% chance for gil, 50% chance the item treasure is the Mirage Vest without the Diamond Armlet equipped ).
The vest is rarely stolen from Dheed and Disma, and it is potentially obtained through the Hunt Club by giving Atak and Stok one Rare Game trophy each, and 25 to Blok; the Mirage Vest can then be bought from the Shifty-Eyed Merchant nearby for 24,000 gil.
In the updated Zodiac versions, the Mirage Vest is not sold in shops, but sells for 8,000 gil. It is found in a treasure in Cerobi Steppe's Feddik River (20% chance to appear, 80% chance for gil, 5% chance the item treasure is the Mirage Vest with the Diamond Armlet equipped).
It is still rarely stolen from Dheed and Disma. It is no longer available through the Hunt Club.
The Mirage Vest is a top tier light armor, giving 55 or 45 defense depending on version, a large 800 HP, and 10 to both Vitality and Speed. These are the highest stats for a light armor in the game, and so Mirage Vest is a good endgame equipment piece. Its HP boost can be very useful, especially under Bubble, and coupled with a light armor hat, the wearer can get close to or even surpass 9999 HP on higher levels; though the user interface caps HP display to 9999, a character can exceed this, best seen when an enemy's attack does 9999 damage to them but doesn't kill them.
In the original version everyone can wear any equipment piece, and thus the heavy and mystic armor may be preferable to light armor, as the HP boost from light armor is not usually worth prioritizing over heavy armor that boosts Strength for physical attackers, or mystic armor that boosts Magick Power for spellcasters. In the Zodiac versions, Machinist, Monk, Archer, and Shikari can equip the Mirage Vest, and so it is only useful to look for if the player has chosen these License Boards for their party. In The Zodiac Age, after fighting Belias, the player can choose two License Boards per party member, and so more party members can choose from the potentially more useful heavy and mystic armor sets. The vest is much rarer in the Zodiac versions however, the best chance being stealing from Dheed and Disma, the latter which is perhaps the hardest non-boss foe in the game and doesn't respawn when killed. The player can fight Disma as soon as they get the key to the deeper parts of Lhusu Mines and they have started the Rare Game sidequest, but by this time the party likely struggles winning against Disma. Were the player to get the Mirage Vest at this point, it would be a superior armor to wear.
Mirages are naturally-occurring, optical phenomena in which light rays are bent to produce displaced images of distant objects. The word "mirage" is of French origin and derived from the Latin mirari, which means "to look at" or "to wonder at".