Uninstall MTA.
Uninstall Vice City.
Delete the Vice City folder.
Install Vice City, no mods, cheats, trainers etc, no patched exe.
Play it once.
Install MTA.
Play.
PS. MTA's current cheat protection (as useless as it is) runs be checking the memory for words like "hack", "cheat", "trainer" etc, so make sure you have NOTHING ELSE OPEN WHILE PLAYING MTA!