Are you using a template for your overlay? If not then you can just view the html source fo the main page while you're logged out and find the part that starts
Code:
<form id="navbar_loginform" ...
and copy to the </form> tag. You also need the line above it:
Code:
<script type="text/javascript" src="clientscript/vbulletin_md5.js?v=420"></script>
The greyed "User Name" that goes away when you clikc doesn't work (I'm not sure why, you probably need abother js file and/or some CSS), so you eithe rneed to figure that out or remove it.