Using /etc/pam.d/vsftpd
auth required pam_mysql.so verbose=1 user=vsftpd passwd=******** host=localhost db=vsftpd table=accounts usercolumn=username passwdcolumn=pass crypt=2
account required pam_mysql.so verbose=1 user=vsftpd passwd=********  host=localhost db=vsftpd table=accounts usercolumn=username passwdcolumn=pass crypt=2

/var/log/auth.log:
Mar 14 18:45:32 scdweb01 vsftpd: pam_mysql - option verbose is set to "1"
Mar 14 18:45:32 scdweb01 vsftpd: pam_mysql - option user is set to "vsftpd"
Mar 14 18:45:32 scdweb01 vsftpd: pam_mysql - option passwd is set to "*******"
Mar 14 18:45:32 scdweb01 vsftpd: pam_mysql - pam_mysql_close_db() called.
Mar 14 18:45:32 scdweb01 vsftpd: pam_mysql - pam_sm_authenticate() called.
Mar 14 18:45:32 scdweb01 vsftpd: pam_mysql - pam_mysql_open_db() called.
Mar 14 18:45:32 scdweb01 vsftpd: pam_mysql - required option "db" is not set
Mar 14 18:45:32 scdweb01 vsftpd: pam_mysql - pam_sm_authenticate() returning 3.
Mar 14 18:45:32 scdweb01 vsftpd: pam_mysql - pam_mysql_release_ctx() called.
Mar 14 18:45:32 scdweb01 vsftpd: pam_mysql - pam_mysql_destroy_ctx() called.
Mar 14 18:45:32 scdweb01 vsftpd: pam_mysql - pam_mysql_close_db() called.