It's possible that it's hidden and you have Windows set to really hide hidden files/folders rather than just make them semi-transparent. You should be able to change that in your options.
Of course, you can avoid the issue by going to Start>Run and typing in the full path and pressing OK. Even if it's hidden, that should take you to the correct spot. Alternatively, you can use Search in your Start menu to find VirtualStore.
As a side note, you can avoid any need to open that stupid folder by just changing all of your mIRC paths (download/log/etc) to a folder that is not in Program Files\mIRC. Send them to the desktop, to your documents folder, to a new folder such as c:\mIRC Stuff\logs, etc. Vista just steals your files if they are saved within the mIRC folder.