Ref the man page for the su command:
su - change user ID or become superuser
su [options] [username]
The su command is used to become another user during a login session.
Invoked without a username, su defaults to becoming the superuser. The
optional argument - may be used to provide an environment similar to
what the user would expect had the user logged in directly.
Additional arguments may be provided after the username, in which case
they are supplied to the user's login shell. In particular, an argument
of -c will cause the next argument to be treated as a command by most
command interpreters. The command will be executed by the shell
specified in /etc/passwd for the target user.
Did your command "vidalia" finish? Were there errors? Maybe if you do the following command, as root, you can see
more error information in detail:
Can't you do a CNTL ALT F1 and get to a Terminal, and login to use the commands like: user, who, finger, and others to
see who is logged in etc..? CNTL ALT F7 will take you back to your GUI.
What is the URL of the guide, blog, or tutorial you were following?