In the GTA-Vice City Directory, there should be a subdirectory called data. In this folder you'll find a file called "Handling.cfg". Change the File name to "Handling.txt" and open it. Here you can change the weight of the car, it's crash damage multiplier, speed, Acceleration and others. The file itself gives good pointers towards which values should be changed, and you should backup any files you intend to edit.
