The password specified in the file is the ispconfig user password. You need to ascertain if its just the file thats wrong, or whether its wider spread on your server.
The password in mysql_virtual_client.cf *should* be the same as all the other config files:
grep password /etc/postfix/mysql-virtual_*.cf
should return ALL the SAME password which *should* be the same as both config files for your ispconfig in the filenames
If they ALL match then the issue is the password is incorrect. I would be surprised if your backend is working correctly in that case.
However, if you establish the password is indeed incorrect then http://dev.mysql.com/doc/refman/5.0/...-password.html
may help you to reset it.
Don't forget you will be resetting the MYSQL ispconfig user password for access to the dbispconfig database and not the admin password inside the dbispconfig sys_user table.
Perhaps the easiest way will be to use phpmyadmin to reset it.
Hope that makes sense.