To Jack a vehicle, approach it and use the REK anywhere on its surface so that the word "Jack" appears in the middle of your screen. A timer will appear, similar to other hacking tasks, after which the vehicle will change ownership to the hacker, and its color scheme will change to show its new Empire. Cloaked Phantasms and Wraiths can be jacked, but requires the hacker to use the Darklight implant to begin the jacking process.
Vehicles may be hacked if they belong to another empire. Advanced Hackers may only jack unoccupied vehicles. Expert Hackers may jack both occupied and unoccupied vehicles as long as they are stationary. All vehicles are hackable, including Empire Specific enemy vehicles that would otherwise be unavailable to the hacker through Vehicle Terminals, and BattleFrame Robotics, although the timer there is long enough to make jacking a tricky process. One-Manned Field Turrets can be jacked with or without an occupant. Nobody may enter a vehicle in the process of being jacked, and those who try will receive a text message warning that the vehicle is being stolen.
The hacker will still need to be certified to drive the stolen vehicle if he is to make a clean getaway with it, although may hop in the gunner seat immediately and start causing havoc. The original owner of the vehicle is immediately notified on his screen that his vehicle has been stolen, and the proximity map icon will change from green to red for all to see, so getting the vehicle away before it is destroyed takes some planning.
At present there is no way to store jacked vehicles in any sort of Lockers. Also, most empire specific vehicles use empire specific ammuntion, which can only then be replaced by stealing another vehicle with a full Trunk.
MAX Suits may not be jacked, and you'd be amazed how badly they react when they get a REK in the faceplate...