Forums Feature Suggestions does mIRC really have to be Vista Compatible?

You seem to be misunderstanding what's happening here. This isn't happening because mIRC is "Vista compatible", mIRC became "Vista compatible" to avoid this issue. mIRC places settings for each user in their user directory by default because Vista prevents regular users from writing to the "Program Files" directory. By circumventing mIRC's default behaviour on Vista you're running up against the problem that mIRC becoming "Vista compatible" was explicitly designed to avoid.

So your issue is, if anything, with Vista itself. There's nothing mIRC can do about it. Personally though I strongly agree with Vista's behaviour. Vista, XP, and 2000 are all multi-user operating systems and many many people use them in this way as "family" computers and so on. In XP and 2000, by allowing regular user accounts to affect system-wide changes these operating systems are vastly more vulnerable to being completely compromised. One person using the computer gets infected with a virus/trojan and suddenly everybody on the computer is infected too. The solution is to have each user only able to write to their own personal directories unless they explicitly provide an administrator password each time they want to do something system-wide. Vista is heavy-handed in its approach to doing this, but only because the subtler hints to get people to use limited user accounts in previous versions of Windows went completely unheeded.

You can still run several versions of mIRC from one user account. You've said you don't want to be told how to do it so I won't bother explaining but it really isn't that hard. It's certainly no harder than it was in previous versions to use a single mIRC installation separately for different user accounts, which is a far more common scenario than the one you're describing. So in this way the new setup of mIRC is a great deal simpler and more user-friendly for the more common usage than before.