Vehicles unlock when you level up (by earning money). To unlock the last car, the Ferrari FXX, you must simply reach Level 49. Upon winning all Career Mode events, you will be around Level 30-35. A great method to boost these last 15-20 levels is use a tip discovered by ISAAC1982 which can be found HERE. This method allows you to level up rapidly, earning roughly $1,000,000 every 20 minutes. To reach Level 49 requires roughly $7-8 million cumulatively earned cash. If using the method above, I found it easiest just to set the phone down (with auto-accelerate enabled) and let your car automatically complete races. This overall is a bit slower than actually playing the levels, but requires only that your hit "Retry" after each event. I found the best and quickest event for this method to be the Athens Time Attack. If not playing, the level only takes about 1 minute and 15 seconds to complete and you consistently earn $75,000.
The "Millionaire" achievement will unlock between Level 40-43 during your grind as it only requires you to earn $5,000,000.