magento login to admin from frontend error:404 error page not found

Hi,
I was trying to login to admin panel by using frontend login template.
Here is my form whose action post to admin login action.

<form action="http://www.mysite.com/bgsh/index.php/admin" method="post" id="customer-form">

I changed the action to :

action="http://www.mysite.com/bgsh/index.php/admin/admin"

and BINGO:

no error on login.(Did you noticed the double admin words in the action value?)

If you request, then i can explain and provide code in details how i manage to login from frontend using vendor credentials, and after logout redirected to the frontend in case the vendor is logged in from frontend.