$mircdir in Vista will point to wherever mIRC.ini is located, wheter that is your mIRC folder or the one in Appdata depends on the user's installation. $scriptdir will point to wherever the script is installed.
If you use $mircdir\scriptname\whatever, and someone puts the script into the mIRC installation folder instead of in the Appdata location, then it will point to the wrong location. I think people need to consider installing scripts into the Appdata location in the future, but that doesn't help for everyone who chooses not to. $scriptdir should point to the correct location no matter how people have things set up in Vista... just remember that you cannot write to the mIRC install folder without changing settings, so if you are writing settings, you will not want to use $scriptdir.
A good option is to use $scriptdir for locating the actual script, and then saving all settings into $mircdir\scriptname\ or whatever. Using a combination of both should allow things to work regardless of how people have things set up... your settings will just not be in your mIRC install folder.
