I just finished this game and although I have had enough money to buy all three of the vehicles' keys, I've held off on it because, being the super-awesome person I am, I wanted to wait until I was playing co-op with someone because I heard that if you do, they get the keys as well, and can keep them once leaving your game.
I just wanted to ask those that have done this if picking up the key yourself gives it to them as well. Do THEY have to pick up the key too? Or does only one person need to pick it up?
So I answered my own question a few days ago. If you're in a co-op game, BOTH of you get the keys. It doesn't matter who picks them up. So long as the 2nd player saves their game, they keep the keys. In fact, the buyer of the keys can then leave the game without saving, then join the other person's game, and they'll receive the keys from THEIR game and still keep the money. XD I did that by accident actually, but realized I got the keys anyway afterwards!