Login and Authentication

[SnappedNet]

Basicly, I'm trying to have use my own login form to login to vbulletin, the only problem is I can't retrieve the password from the database because of different levels of encryption.. How I do decrypt the password?

This is my current string:

PHP Code:

$qry="SELECT * FROM sn_user WHERE username='$username' AND password='".md5($_POST['password'])."'"; 


What do I change in the password='".md5($_POST['password'])."' to retrieve the password from the database correctly?

OR

How do I edit the login page so I can make it look similar to this: