Have you tried:
DirectoryIndex index.php
More resources on that: http://httpd.apache.org/docs/2.2/mod/mod_dir.html
In your case:
<VirtualHost *:80>
# (other directives)
DirectoryIndex index.php
</VirtualHost>
Update
As jcjr
suggested, this directive can be also put inside .htaccess
file, but ensure, that you've enabled such method by proper AllowOverride
directive value: