vba will allow you to easily be in the same directory instead of putting vbulletin in another directory. Just change the name of the vbulletin index.php file to forums.php (or whatever) and also change the name in vboptions > forumhome options.
But anyway, I'd suggest learning how to deal with the vbulletin options before you go and install vba. Why do you want vba? I ask because so many new owners just throw that onto their forums and leave it at default (they don't change the appearance at all) and it's kinda a useless page.
everytime i changed the index thing my forum went down.