Looking at the situation of yours I am suggesting that you should check the /etc/pam.d/vsftpd on the computer of yours. also you should try with the below mentioned setting and see whether it is working or not.
Also you can refer the pam.d/vsftpd file of mine to get help from the same.
auth required pam_listfile.so item=user sense=deny file=/etc/vsftpd/ftpusers onerr=succeed
auth required pam_shells.so
auth include system-auth
account include system-auth
session include system-auth
session required pam_loginuid.so