Virtuser file, you'd have to look it up on your own. I'm not master, and wouldn't want to lead you down the wrong path.
Using the full email address as the login is the default for Rouncube. So no modificaitons or examples should be needed. Just input your full email (user@domain.tld) for the username, and your password for the password.
For setting the default domain, just open up config/main.inc.php and change the following setting:
// the mail host chosen to perform the log-in
// leave blank to show a textbox at login, give a list of hosts
// to display a pulldown menu or set one host as string.
// To use SSL connection, enter ssl://hostname:993
$rcmail_config['default_host'] = '';
Between the single quotes, just put your server.
Example:
// the mail host chosen to perform the log-in
// leave blank to show a textbox at login, give a list of hosts
// to display a pulldown menu or set one host as string.
// To use SSL connection, enter ssl://hostname:993
$rcmail_config['default_host'] = 'imap.myserver.com';