Back up the config file before making a change;
sudo cp /etc/vsftpd.conf /etc/vsftpd.conf.back
and then edit vsftpd.conf (with vi or nano)
nano /etc/vsftpd.conf
Then make the following change
pam_service_name=ftp
Save your change and restart the ftp server (if you use nano hit CTRL+O & enter to save then CTRL+X to exit)
sudo service vsftpd restart