It's not Vista, it's how mIRC was written.

Originally Posted By: "versions.txt"
20.For Vista compatibility, new installations of mIRC now save all
settings in the User's "Application Data\mIRC" folder by default.

However, if the installer or mIRC find mirc.ini in the same folder
as mirc.exe they will use the mirc.exe folder for all settings.
Users installing over an old version of mIRC should therefore see
no change in behaviour.

The installer is now also aware of administrator and standard user
accounts and installs mIRC accordingly.