How to add username or change password in vsftpd in Ubuntu 16.04?
By default vsftpd users are the ones of the system. So if you have admin access, root user or a sudoer, you can log in, view the users using getent passwd command and passwd SomeUser as root to change her/his password.adduser SomeName to add a user.
Check also for virtual users for vsftpd. Inside vsftpd.conf file search for something like:pam_service_name=
and then locate this file name inside /etc/pam.d/
In case PAM mechanism is confusing you, a good indication of virtual users is virtual_use_local_privs=
inside vsftpd.conf.