The guy in this question has a similar issue and configuration. The correct answer there is to comment out the following in your /etc/pam.d/vsftpd
auth required pam_shells.so
vsftp login errors 530 login incorrect
The guy in this question has a similar issue and configuration. The correct answer there is to comment out the following in your /etc/pam.d/vsftpd
auth required pam_shells.so