It depends how the front page is coded.
If its in PHP you could edit the PHP
PHP Code:
// #########################
// TOP OF THE SCRIPT
// #########################
$curdir = getcwd();
chdir('path/to/forums/');
require_once('global.php');
chdir($curdir);
// #####################
// Where ever its meant to go
// #####################
if($vbulletin->userinfo['userid'])
{
echo 'Welcome! Bla bal';
}
else
{
echo 'LOGIN FORM';
}