Download and install the most recent update patch for "Grand Theft Auto: Vice City." Sites like CNET have archived the patch on the Internet for free download. Download the file and double-click it to install the patch. Close the game before installing the patch. The official patch is designed to fix certain graphical issues, like lag.
Open the "Settings" menu in "Grand Theft Auto: Vice City" and examine your current settings. The lag may be resulting from a setting that isn't compatible with your computer. For example, open the "Screen Resolution" menu and adjust the screen resolution of the game to a lower setting than the one it is currently set to. Try to play the game again and see if the lag is fixed.
Use "Programs and Features" to uninstall any add-ons or mods for "Grand Theft Auto: Vice City" that you've installed from third-party developers. This application is in the "Control Panel" in Windows. Certain people unaffiliated with Rockstar Games (the makers of the game) have released additional content that can be downloaded and installed. However, this content may not be completely compatible with the game and can cause lag during regular gameplay.
- You may not be able to completely eradicate lag without upgrading your computer's hardware.