The easiest way to do this that I can think of is to have two login modules.
One of them for "public" access, and the other for registered. That way you could have them in different locations.
If you go that route, the only other thing you would need to do would be to edit the template so that the top module does not get loaded if a user is logged in.
<?php
if($my->gid){
//do whatever you want to do for a logged in user
}
else
{
//user isn't logged in, so load module as usual
mosLoadModules('top', -1);
}
?>
Please note that I have not played with Vortex yet, so 'top' might not be the correct module location. Also note that this code should replace the existing PHP code inside the div where the login is displayed.