I just tested this by creating a limited user account on my Vista system- it worked fine.
Exactly what is the error message you're getting that makes you think it won't work from a limited account?
One potential problem I can imagine is if you have the mirc.ini file in your C:\Program Files\mIRC folder. This might cause permission errors from a limited account. The correct way to handle multi-user setups in mIRC from vista on is to store the mirc.ini in the AppData folder of the respective user-- this is the default setting in a clean install for mIRC 6.3 onwards, but if you upgraded from a pre-6.3 version of mIRC, you'd need to (re)move the mirc.ini file from program files to get proper multi-user functionality.
