Ok, look:
You should disable UAC if you play games a lot, it's a bit of a security risk but if you have an antivirus/firewall you should be fine. Disabling UAC will let you have administrator privileges over everything, so I suggest doing that. Here's the link for instructions: http://www.petri.co.il/disable_uac_in_windows_vista.htm
After you do that, I suggest completely reinstalling Vice City/VC-MP, I mean completely. First just uninstall it - at the end if it asks you to restart, you should restart. After that go to C:/Program Files/Rockstar Games/ and delete the GTA Vice City folder there permanently. After you do this download CCleaner: http://ccleaner.com/ and run the cleaner part of it with no programs running. After that click the "Registry" tab on the side, click "Scan For Issues" and after its done click Fix selected issues... It will ask you to make a backup of the registry, do it in case anything fucks up. After all of this is done restart your computer and install Vice City the way the installer shows you too. Do the full installation and not anything like Custom, just do the full. After that I want you to make a backup of Vice City. Go to C:/Program Files/Rockstar Games/ and right click "GTA Vice City" or whatever the folder is called and hit "copy". After that go to your Documents folder and hit Ctrl + V and let it copy the files into there. Next you may install VC-MP. Post here your results.
Please note that VC-MP's GUI windows aren't coded for vista. In order to close the VC-MP window you need to CTRL + ALT + DELETE and end the process vcmp.exe, or else you'll get crappy runtime errors.
By the way, if you had Multi Theft Auto installed, VC-MP conflicts with it. I mean, it just doesn't run, even on XP. Pick from one of them and install. Post your results.