Here's a backup script I use for network servers I run under Vista. Save this as backup.bat and put it in your dominions install directory, in the same folder as dom3.exe. If your games save in a different directory than default in Vista, you'll have to change the path in the script.
Code:
set game=%1
set gamebackup=%game%Backup
xcopy /c /y /i "%USERPROFILE%\AppData\Local\VirtualStore\Program Files (x86)\dominions3\savedgames\%game%" "%USERPROFILE%\AppData\Local\VirtualStore\Program Files (x86)\dominions3\savedgames\%game%\%gamebackup%"
Add this to your hosting shortcut:
--preexec "backup.bat GAMENAME"