Forum Modifying the index page of vBulletin
 

I wanted to create a main website for my vBulletin forum, but until the entire site is ready I just wanted to edit the index page to a type of 'Under Construction' page with a link to the forum. To do that, which is the file I need to edit, and where can I find it? Also, would I need to create a new file with the same name and overwrite it or edit it through the File Manager since Filezilla doesn't allow you to edit files?

Also, for when the entire site is ready, how would I have it as different website under the same domain file-wise? Would I need to create a new file directory in public_html or how would it work?