Once you've built a Vehicle Pad, you'll need to spawn The Vehicle on it to drive around. To spawn your vehicle, walk up to the Garage/Vehicle Pad and hold the E button. Your vehicle will spawn on the pad.
If you've already spawned a vehicle, driven it around, and lost it or left it out in the world, simply walk up to your garage and hold E again. The vehicle will respawn on the pad.
To enter or exit the Vehicle, walk up to either door and hold E. Driving the Vehicle is the same as normal movement (WASD).
Your vehicle has its own O2 and Energy stores. When you're inside it, you will deplete the vehicle's O2 first. When that's gone, you will switch to using your Suit's O2. To replenish the Vehicle's O2 supply, park it on the Garage/Vehicle Pad and let it recharge.