Thanks to NSanityHD, I've learned that saves from the Mac and Windows versions of Vice City are not compatible with each other by default. After looking at the Mac version, I've found that the difference is the scripts (main.scm). Much like San Andreas and v1 and v2 scripts, you can swap your Vice City scripts to load saves made with other scripts. Other than the scripts, the Mac version appears to use the same save format as the retail version for Windows.
So the following changes have been made to the Vice City (PC) download pages.