These are several unlockables that you can get an access to under the following conditions:
Compact Series:
Randome Compact Collector: Awarded for Gold in Silver Lake Eliminator
Dominator Compact: Awarded at 10,000 Burnout Points
Tuned Compact: Awarded for 5 Gold Medals
Legend Compact: Awarded for Gold in Interstate Loop Legend face-off
Gangster Legend: Awarded for Gold in Palma Bay Legend face-off
Assassin Compact: Awarded at 15 Takedowns
Muscle series :
Tuned Muscle: Awarded for 20 Gold Medals.
Assassin Muscle: Awarded at 30 Takedowns
Randome Muscle collector: Awarded for Gold in Harbour Town Eliminator
Muscle Cop: Awarded for Bronze in Harbour Town or Sunrise Valley Pursuit
Classic Legend: Awarded for Gold in Big Sure Shore Legend face-off
Legend J-Muscle: Awarded for Gold in Airport 1 + 2 Legend face-off
Dominator Muscle: Awarded at 25,000 Burnout Points
To to unlock Special Vehicles, you have to play through the game on World Tour and get gold in all mentioned events:
Gangster Boss: Race events
Cop Racer: Pursuit events
Firetruck: Crash Events