Here is a tested method of putting a vehicle in a first person shooter. It works in sort of a round about way. I have tried all these methods, and they work fairly well.

Method 1
: Weapon replacement/ Level switch. Have an md2 of a vehicle placed in front of you. Have a level switch occur right before the vehicle. In the second level have an animation remove the players weapons and a prompt to press enter in order to get out of an invisible box with one wall a door that requires a certain weapon to get out; the door should go up not side to side. Have the floor of the new level like ice to simulate a slowdown rather than a sudden stop. Make sure the weapon that the vehicle is replaceing is a weapon you don't intend on useing in the game. make sure the vehicle causes no damage when fired to avaoid awkwardness. Also increase the players speed for this level to simulate the speed gained. Have the destination point highlighted with an animation that prompts the player to enter here and at that destination point have a level switch to the next level. in the next level have the player standing next to the vehicle with an animation that removes the particular weapon, and another that gives the players weapons back.
Down side
: The amount of ammo will not be the same for the weapons recieved back. Chalk it up to "there was ammo in the car"

Method 2
: When an object that causes speed change or jet pack type capabilities is picked up, have a console object drawn to simulate the front of a vehicle with a vehicle console. Ofcourse have the object that is picked up resemble the vehicle to be driven. When the vehicle is dropped the effect is gone.
Down side
:It's notthe best looking effect as the console is drawn ontop of the players weapons, and pieces of the weapons may show under the vehicle's console. Also a solid black vehicle will not be solid black due to black not being drawn on console objects.

I will post pics of these methods in action when I get a chance.