I've spent the last day or so looking into this and so far I haven't been able to find a solution.
It seems that there is no easy solution to this bug, as Khaled himself wrote in this thread about 9 months ago. This probably won't be fixed until mIRC goes completely natively unicode, since that's the heart of this issue and is likely the only real solution. Assuming Khaled exhausted all options, it seems as though Vista does not expose enough API to do anything about this specific issue. Frankly, I would say that this is really a Vista bug, not an mIRC bug since there is no real reason why it would revert to a non-aero theme in this situation
(edit: It may not be logical, but Microsoft may have intentionally changed the way to custom draw the titlebar in Vista).
Perhaps some references on the issue might help K find a solution, since others have indeed had this problem and (I assume) solved it:http://www.ureader.com/msg/14721474.aspx