The ftp user is a non-interactive account, used for running server processes. The login users to your ftpd server is what you must control.
If I understand you (put ftp under their jail directory) you want to chroot the ftp connections. Then, use the chroot_list_* directive from /etc/vsftpd/vsftpd.conf.
Finally, if you only want to use SSL on ftp, then only allow connections through port #22, and use sftp as a client.