The Vault-Tec Assisted Targeting System, or V.A.T.S., is a special feature for those who obtain a workable Pip-Boy. The Pip-Boy helps by granting a bonus to attack a creature and allows you to focus your fire in a specified location. 

When using V.A.T.S. against a humanoid enemy, seven different areas can be targeted: the head, torso, left arm, right arm, currently held weapon, left leg, and right leg. However, there may be other points available when facing off non-humanoid creatures (e.g. robot combat inhibitors, ant antennae).

Activating V.A.T.S.

Activating V.A.T.S. requires a Bonus Action and increases your chance to hit with any weapon by +5. You can use any amount of your Action Points while using V.A.T.S. (ex. if you have 8 Action Points you can make 2 attacks with a 10mm pistol since its damage dice is a d8.) 

Action Point Cost

The cost of using a weapon in V.A.T.S. is equal to half the damage die the weapon can do. Ex. The 10mm pistol damage die is a d8, each shot in V.A.T.S. would need 4 Action Points. 

Reloading a Weapon in V.A.T.S. takes half your maximum action points.

Action Point Regeneration

You recover action points at the start of your turn equal to half your agility rounded up. This can be increased with Perks such as Action Boy/Girl.