You will need multiple playthroughs to grab this achievement. There are vehicles that appear in the decision towns meaning you cannot get to both in one playthrough.
The following vehicles are optional:
Small Sedan found in Fontana. You should pass the car on your way to the diner. In order to get the keys you need to do the optional objective involving Mia and Scout. Return back to where Scout would be in order to get a set of keys.
Crossover found in Lemon Hill. The SUV itself is found down the back alley behind the gas station where Merle is. The keys for this vehicle are found in the room overlooking the bus that has the ladder extending from the building to the top of the bus. They are on the nightstand next to the dead person.
SUV found in Lafferty. The keys are found in the same room as the poster. When proceeding through the grocery store, a wooden door will appear to be locked until a walker comes out of it. On the desk in that room are the keys. The SUV itself is in the final stretch of the mission right before jumping ontop of the police car to finish the mission.
Double Cab Truck found in Archer's Creek. The keys are in the same building as the squirrel and poster (the second building on the left when you push the last car. The keys are on the shelves in the first back room on the right.
Pretty simple..doesn't need 1 playthrough can be 2 or as many as you need..once you find 1 of them and completed the mission..you do not need to go back in order to get the achievement.When you find your final key..the achievement will than pop.