I've had mIRC on Vista for almost a year now and don't really notice any issues. Dialog size and control size/coordinatesmay be affected. Meaning that a dialog madeon XP may not look the same on Vista.

Make sure mirc.ini is in your root folder or your path may point to your users\yourname\appdata folder. This isn't a bug, it is intentional.

Maybe these sites considered that some people use mIRC to flood with or hand out trojans. Which is silly since the actual mIRC.exe doesn't have any of this.