There aren't a lot of bugs with Vista. However, those that are there can affect people enough to cause problems for them. Depending what you do with mIRC, you may never experience problems, or you may. Reading the bugs for Vista would tell you if you'd have to deal with them.
That said, unless you have problems with your current OS, why upgrade? We know there are problems with mIRC and Vista. We know there are problems with various other software and Vista. We also know that there aren't really any major changes in Vista other than security and some GUI changes. If you're running fine in your current OS, when shell out hundreds on something that could give you problems and isn't really an upgrade? Just my opinion.
