Warthog. The Warthog has three positions: Driver, gunner and shotgun seat. They should usually be filled in that order. Each provides a separate role, and each is important to the safety of the others. Try to have one of each when possible.

Driver: Driving the Warthog is the equivalent of having an empty plasma pistol. It can't shoot, but it's a really fast melee weapon. Running people over is fun and useful. Sliding sideways into them gives you a better chance of hitting them than if you run at them head-on; it gives you a wider area and less opportunity for them to dodge. Get in the driver's seat with an appropriate weapon. If you're being harassed by an infantry with a rocket launcher, it's often useful to bail and use a rocket of your own on them. The same is true with another Warthog. As long as they're stopping for a fight, jump out and nuke them with a rocket. On the other hand, if they throw a plasma grenade on the jeep, a shotgun can be useful as well, as they will usually have one of their own. This is the best position for the flag-bearer, as it can be seen if the passenger has it and the gunner is vulnerable. Remember to hit A for brakes; this is useful for sliding into turns.

The Rocket Warthog (computer versions only). The Rocket Warthog works and handles exactly like its LAAG-equipped counterpart, but with a three-barreled rocket launcher on the back. These rockets are roughly equivalent in power to a SPNKr round, but you can fire a lot faster. The Warthog gives you a height advantage as well, giving you the ability to fire at an enemy's feet with ease. It is best to close with another Warthog before attempting to fire, as it is hard to hit a far-off enemy; wait for them to get fairly close and then launch at the nearby ground to flip them, and then run the survivors over.

A word of warning about bad gunners: If someone gets in your jeep as you are driving and immediately empties a tube or two at nothing, just bail. You're screwed anyway. Ask him to drive if he's any good, and if he's not just find another jeep. TKing is not recommended. But if he's going to waste ammo like that (reload is long) he's not worth having on the back of your jeep. This goes for you too- don't fire unless you have a target. 

Scorpion. The Scorpion tank is slow, lumbering, and immensely powerful. It makes a good target, though, so watch your back. It also supports up to 4 armed passengers; they are more vulnerable than the driver, so ride at your own risk.

Driver: Be careful where you go, and watch for snipers. The Scorpion is best for short range combat; due to the immense size and wide field of view of the tank, targets seem pretty small onscreen. The shell is faster than a rocket from the launcher and does more damage, but it is slow to reload. The tank's secondary fire (grenade button) is a coaxial machine gun, the mounted equivalent of the AR. If you miss, or are being surrounded by multiple targets, use the coaxial gun to wear down on their shields while you reload the main barrel. The Scorpion's best use at long range is shelling an enemy base. Have an armed passenger carry a sniper rifle and spot targets; when he fires, follow his contrails and fire the tank's main gun. It is fairly accurate at long range, and fast, as long as you can see what you're shooting.

Side Seat: The side seat is useful for assault transport. Take a bunch of people, approach the enemy base, and blast the base while your people unload. A passenger with a sniper rifle can call in tank fire from the main gun. Get off if combat starts, as the passengers are vulnerable.

Ghost. It's kind of like a flying motorcycle with an armored windshield. And guns. The guns will stun your opponent as you approach, making it easier for you to run him over. Strafing is also handy; the Ghost maneuvers just like you would if you were on foot, and it's faster, so keep in mind that you can slide into a target from the side. Clipping them with your wings will kill just as well as any other hit. The A button lifts the nose, which is useful for ramping rocks, getting extra air, or pulling out of a nosedive. When approaching another Ghost, strafe-circle around them to throw off their aim; the same will work against someone on foot. Careful about maneuvering in tight spaces-- the wings will throw you off track.