capturing tanks and helicopters

Well,first you have to do the mission where you fight the Vehicle
Then,you have to neutralize all of the Vehicle's Soldiers,you don't have to kill them,try putting them to sleep/knock them out and Fultoning them out,or lure the Vehicle to run over/Attack them,so it's not you that killed them
If you managed to eliminate ALL Soldiers(No more Soldiers show up)without getting detected by the Vehicle,the Commander will Pop out of the Vehicle's Hatch,Tranquilize him and you have yourself a new Vehicle
However if he Vehicle has already Detected you,the Commander will not pop out once all the Soldiers are eliminated,you will have to damage the Vehicle by 3 Bars(Reduce 3 Bars of it's Health)so that he will come out,you also have eliminate all the Soldiers that keep showing up first,then he will pop out,Tranquilize him,New Vehicle

It's easier said than done though

Like I said,lure the Copter/Tank/APC to Kill their own Soldiers,hang around the Knocked out/Sleeping Soldiers and wait for them to shoot at you,for APCs though you'll have to make them run over their Soldiers,since Machine guns never seem to hit their own soldiers,Helicopters are the easiest,just make sure they drop Bombs/Shoot Missiles at you when you're hanging around the Knocked out Soldiers..Of course this requires you to be detected by the Vehicle

Short answer, eliminating the entire sqaud (10 - 30 people). If you haven't been spotted by then, the Captain should pop his head out of the vehicle and you can take them out too. Otherwise, you have to damage the vehicle (after eliminating the entire squad) enough to make the captain/commander appear.

There is a device called the EM Net that you can use to capture land vehicles. I forget how you get it exactly though.