Then I point out some basic commands to add USER, and to change the password to a user, in vsftpd:
adduser -m -p clave -s /bin/false usuario (user to create and establish key)
adduser -m -p password prueba
user passwd (to change key)
More information:
http://www.redes-seguridad.com.ar/2008/03/servidor-ftp-seguro-vsftpd.html