Note: do this in either Jimmy's Vendetta or The Betrayal of Jimmy. First, start the mission with the vehicle you would like to keep. Once you are inside the vehicle, get back out and get into another car (preferably the one you started the mission with). Get back out and into the target vehicle once more. At this point, complete the mission. At the scoring screen, select to REPLAY the mission (not continue). The replay should start with the target car as your vehicle. Drive it to a garage, store it, and then complete the mission again. Now you have the vehicle in your garage, and can use it as you like! This also allows you to acquire cars that are impossible to find elsewhere, such as the Culver Empire in the Betrayal of Jimmy and many 1940's-only cars in Jimmy's Vendetta, as well as the Prison Bus (although the bus can't be stored in a garage).