Add Facebook Connect to VirtueMart Login Module

I'm not familiar with the exact code of the VirtueMart login form, off-hand, but can look into it further if necessary. Generally, the file you'd want to modify would be something like /modules/mod_virtuemartlogin/tmpl/default.php. However, Virtuemart doesn't always follow Joomla standards, so it may not be that simple.

Another alternative that may work for you is to create a Custom HTML module. Put it in the same position as your VirtueMart login module (either above or below in the ordering) and in the text area, put:
Or quickly login with Facebook:
{JFBCLogin}

Depending on your template and site, this may or may not flow with your site. Hope this helps, but again, if you need more help, let us know. I can look into how to do it in your situation, but it may take a day or so to get you an answer.