1. You need to go to your Vice City install directory (usually C:/Program Files/Rockstar Games/Grand Theft Auto Vice City) and when you're in the VC folder, look for a folder called "data" and double-click it.
2. Find a file in there called "handling.cfg" and open it with notepad.
3. Search for the Voodoo car's line, select the entire line (except the name) and copy it.
4. Then find the car you want to replace, delete it's entire line (except the name) and paste the Voodoo line in.
5. Save the file and start VC.