Own every vehicle in the game

You can buy a new vehicle when you start a race. When you are in the car select menu scroll all the way to the right to find cars for purchase. Note that there's not always a new car to purchase though as you have to unlock new cars by completing events first.

There have been reports of people not getting this trophy/achievement even though they've bought all the cars in the game. The only known solution to this issue as of now is to delete your game/save data and play through the whole Showdown Tour again. I advise to make a backup of your save file before you buy you're 30th car