[Minots]

@trafix
in your current .htaccess file, is there any line begins with
DirectoryIndex? it works & look like the one from the apache configuration-file (httpd.conf).:
i.e.:

DirectoryIndex index.html index.htm index.shtml index.php index.php4 index.php3 index.phtml index.cgi

if this line is already present, you need to change it like.:

DirectoryIndex vbsubdomain.php index.html index.htm index.shtml index.php index.php4 index.php3 index.phtml index.cgi

if you don?t have it in your .htaccess -> simply add it