Prevents sneak attacks and back attacks.
Vigilance (けいかい or 警戒, Keikai?, lit. Warning), also known as Caution and Alert, is a recurring ability in the Final Fantasy series, associated with the Thief job class. It generally prevents back attacks.
Prevent back attacks.
Vigilance (also known as Caution) is a support ability for the Thief job class, costing 75 ABP. It reduces the chance of a back attack on the party from 1/16 to 0.
Vigilance is found on the relic Alarm Earring which protects against Back Attacks and Pincer Attacks. Because some enemies only attack in certain formations, this eliminates some Random encounter groups.
Although not mentioned in the Materia's description, the Preemptive Materia, when mastered and placed on the last character on the party, gives an "Ambush Alert Bonus", which halves the chance of back and pincer attacks.
Each area has a set number of battle formations that are possible to occur during a random encounter. When a random encounter is about to start the game performs a check to see if any of the "special attack formations" apply. If the area has any special formation battles (Back Attack, Side Attack and Pincer Attack) the chance of getting them is calculated first. These battles have the player at a disadvantage, but a mastered Preemptive Materia helps protect against these battles by halving the chances of the disadvantageous ones.
The game uses two types of check in order to detect whether of not a party member has a mastered Preemptive Materia equipped: a single character check and a global check. The single character check is made whenever the player changes Materia or Equipment. If the character does not have a mastered Preemptive Materia the game will remove the Ambush Alert Bonus, which halves the chance of Back and Pincer Attacks, and it doesn't matter if another character does actually have a mastered Preemptive equipped.
The global check is used when entering the menu or exiting the PHS screen by running the single character check on each character in the party in turn, starting from the top. What this means is that only the bottommost character in the party counts for whether they have a mastered Preemptive Materia equipped.
Prevents Back Attack.
Alert is available from the Guardian Force Cerberus for 200 AP. It doesn't protect against fixed back attacks, such as the one in Deep Sea Research Center against a Ruby Dragon whilst answering Bahamut's questions.
The Alert support ability costs four Magic Stones to equip and reduces the chance of being ambushed from 24/256 to 0%. It is learned from the Barbut, Ninja Gear, and Germinas Boots. Alert can be learned by Zidane (40 AP), Steiner (60 AP), and Amarant (30 AP).
Alertness is a job trait for the Ranger at level 5. Its description says that it "Lowers the chance of being detected by enemies." Nothing about how this works is known with any certainty.
Assume a defensive stance.
Vigilance (also called Caution) is a reaction ability. The Thief class learns it for 200 JP. Receiving an enemy's attack causes the character to take up a defensive position that doubles the unit's evasion rate (but not reduce damage received). It lasts until the character's next AT.
Vigilance is the level 12 ability of the Thief class, requiring 230 AP to learn. It requires 1 slot in order to equip and use. The ability prevents sneak attacks and back attacks.
Vigilance is a special ability learned by Onion Knight, Bartz, Faris and Locke. Vigilance activates once per stage during Field Music Sequence, and prevents the character from being replaced if they fall.
Vigilance is a reactive ability that costs 4 CP to equip. It is activated just once in Field Music Sequences (FMS) when a character is hurt. The character will not be switched out for the next party member like usual. It is learned by Onion Knight (level 5), Bartz (level 5), Galuf (level 10), Sephiroth (level 5), Vincent (initially), Tidus (initially), Vaan (initially), Balthier (level 5), Rikku (level 15), and Zack (level 5).
Backstep out of harm's way, becoming invisible and reducing malice.
Alert is a level 3 Thief ability that costs 3 SP to use and requires 260 JP to learn. It finds enemies, traps, and items on the floor.