The default users are still administrators under Vista but under the control of UAC(you're a supervised admin).
Reads from program files is allowed and is where Windows first looks, it is only if you try to open the file with write access that Windows will direct the program silently to the VirtualStore. Often programs start without write access to files they are accessing and swap to write access only when it needs to. So you'll find mirc reads the "real" file until you try to modify the file and all of a sudden mirc will be reading and writing in the virtualstore(unknowingly) instead, yet on next load it will be back to reading the real file that was not updated etc.
VirtualStore only exists to try and help allow a few more programs limp along under Vista until they are updated to be fully compliant with multiple/limited users
You can create folders in the root of c: (but not actual files).
c:\mirc\ would have no UAC restrictions and full read/write for administrator accounts is allowed. Altho if that programs tries to access program files/HKLM etc, those actions will still be virtualized.
Ofcourse the folders are dynamic, the ones that count under UAC are %homedrive%(root only), %programfiles%, %windir% and %allusersprofile%. Also in the registry HKLM is protected and virtualized.