Like it has been mentioned in this thread, the vehicle unlocking is fine how it is now. Both Banshee and Comet are pretty good cars before unlocking the higher-end sport cars near level 100. Not only that, but they are also cheap to replace if either gets blown up. Lastly, there are several ways to quickly grind to level 100, for example, by fishing, daily objectives, achievements, collecting collectibles, seasonal events, etc.