Can you give the full path (/components/com_virtuemart/????/default_login.php) to the file you edited? That will help. Also, you may want to check for a template override of that file, which, if exists, should be edited instead of the original.
So, for example, if you're editing /components/com_virtuemart/views/user/tmpl/default_login.php, look for the default_login.php in:
/templates/<YOUR_TEMPLATE>/html/com_virtuemart/user/default_login.php
If it's there, that's the file you should be editing as that's the one being used to generate your page.
Hope that helps, but if not, let us know.
Thanks,
Alex