Vehicles can be added to a specific brick called a vehicle spawn. A normal vehicle spawn is a flat brick with a black triangle on it. The black triangle depicts the direction a vehicle wil be facing when spawned.
To add a vehicle to a brick, use the Wrench on a vehicle spawn. A menu will appear containing a drop-down menu of vehicles available. Select one of the vehicles then press "Send >>" to spawn it.
Here are a list of vehicles that come with the game by default: