First, you'll need the list of Vehicle ID's, so check out the following thread. ( https://theindiestone.com/forums/index.php?/topic/23965-vehicle-spawn-ids/ )
The command to add the vehicles is as follows:
/addvehicle "Base.VanSpiffo" "PlayerName"
or
/addvehicle "Base.VanSpiffo" "x,y,z"
First parameter being the Vehicle ID, and the second parameter being where it will spawn, either coordinates provided, or next to the player you specify.